Criticism:
1. I think that the encounters are too formulaic. Every quest is "go to a place, advance through the obstacles, defeat big boss, take prized artifact, go home" and each obstacle is either a trap or a combat encounter or sometimes a riddle, and each combat encounter is simply a standard "fight in the room until you defeat them". There are of course exceptions but the trend is too strong.
For example, there are no combat encounters in which we actually move around a large changing area or use the environment to our advantage (the closest thing to it was the fight against the big terminator robot, or perhaps the fight where I nearly suffocated us all inside the burning carpet room), and there are no combat encounters where we have to protect someone or something else or where the best way to win is to retreat or to steal something and run or to delay for a known reason.
Generally we never need to think much about our movements in combat, beyond getting away from danger and towards our attack range. And generally we all have the exact same jobs - kill all enemies and/or heal/buff each other. We never get to have encounters during which one of us has a special task to do, or where we have personal goals that don't exactly align with the party's goals, or where we actually make a detailed plan with specific tactics and orders for each player.
Moreover, we basically solved death by letting people drop to 0 HP and then healing them as soon as possible. On the other hand, our enemies NEVER heal each other this way, and they don't even run away when they're about to be defeated (neither are we, since we.... never lose any battle). I think it would be more interesting, for example, if we actually had to prevent enemies from being able to heal each other, or running away. Or if we had to think about status effects more (slowed down opponents, flanking?, being prone to evade ranged attacks, casting Silence to prevent vocal-component spells, etc.).
Damage types barely matter to us. Last session we fought against two monsters with resistance to magical damage, and we just did not care at all about that. The only time we had to actually care about damage type was when we fought Moloch and couldn't kill him without Radiant Damage (which was cool). I think immunity is more interesting than resistance, and so is vulnerability.
2. The time management isn't good. You probably know this already but I dislike it when we need to roll 8 times because there are 8 enemies, or when we have to roll extra dice all the time. I also don't like private chats that are taken during actual play unless they're very short, since it basically feels like a time-waste for the players outside the private chat, and roleplaying one-on-one feels weirder and also less "worthy" to me personally (like... only 1 person can enjoy listening to what I do or what you do, instead of 4 people).
In the same way, I don't like it when you spend time on setting up something (usually in the map, e.g. a spell effect) and the game just pauses during that - I think it would be much better if we just continued playing normally, with you working to make the thing happen with your hands while still talking to us about what's happening in-game. Alternatively, if it really requires your concentration, give us something to do with each other (e.g. divide loot or plan an encounter or just roleplay).
In general, it feels like the combat is taking longer and longer, and mostly due to things that could definitely be speeded up.
The obvious culprit here is Oladan, who has to manage 2-8 undead minions along with frequent uses of Minute Meteors and along with thinking about his turn only during his turn. I think it's already getting better (he's using Minute Meteors less now, and we changed it to only throw 1 meteor per turn; he switched from 8 skeletons to 2 poltergeists). I still think it would be better if all of his minions rolled a single combined attack roll, and better if the minute meteors didn't deal half damage when missing, or didn't hit multiple opponents, or targeted AC instead of requiring dex saves. Still, it's better now, since the poltergeists can move through everything and only have a simple melee attack (which is a bit OP IMO but whatever).
Ullad also has slow turns but that's due to lots of attacks and rolls (3 attacks, adding 1d4 cold damage each time, Divine Smite, having to sum up 9 numbers in our head and Overdrive Armor forcing us to calculate maximum damage possible instead of using Roll20 and still sum up numbers in our head, and Bless giving everyone another manual 1d4 roll frequently. Also, Nick sometimes doesn't know what to do with his turn and lots of times will try to do the tactically-wrong thing (e.g. buff us instead of dealing a killing blow). No idea how to solve this. I think it would be good to tell him upfront what his choices are during his turn, i.e. "Nick, it's your turn. You're not in range to hit with an attack, but you can probably cast a helpful spell, or move closer to the enemies. You can also switch weapons if you have no spell you want to cast."
Kiaran usually has quick turns (mostly damage, healing, and tactical movement), and as Shiska I try to keep my turns as short as possible (although sometimes I do think too much time during my turn). I always feel envious of the other players for having useful Bonus Action and Reaction spells/abilities, while I am usually only using a single Action and that's it. But that's not really your fault :P
However, we should probably never again have a battle like the last one we had in Villa Price against shadow monster demons. We were Oladan, Kiaran, Ullad, Shiska, David, Kelsey, ~10 skeletons, and ~8 flying knives, against 5 monsters of different types. This battle took AGES. A-G-E-S. It was fun, yes, but we really could have done it in half the time without losing anything crucial.
In general, abilities that are just attack rolls are the fastest. Someone says they cast a cantrip or stab with a sword, they click the button, they see the d20 roll, and the GM says "it hit" or "it didn't hit" (usually this is clear without the GM even). Attacks that require saves are the slowest, since after the first party announces what spell/ability they use, we have to wait for all of the affected creatures to roll a save. Roll20's UI makes these rolls more cumbersome, and for a DM that needs to roll 4 dex saves after every fireball and then has to wait 15 seconds for Roll20 lag to stop, this is too much.
3. Another personal opinion but - I think you break the fourth wall too often. Or perhaps just inconsistently. I think we should either know the exact HP of the other players (because it doesn't really feel like not telling Ullad how much HP to give us is adding to the fun), or not know the HP of enemies (like, when you tell us "you shouldn't bother with a Fireball, a simple Magic Missile will kill this dude").
In the same way, you telling us about items we're about to have available in shops and "ultimate <class> object" feels weird to me. Stuff like talking about obstacles we skipped or showing us trap blueprints after we passed them is fine, though.
4. You're too mean to Steve, sometimes.
5. For a world where resurrection spells don't work, we sure do have a lot of resurrections..!
6. I think you should get a better computer, or browser, I don't know which. Lots of times you're complaining about Roll20 lagging, but from my perspective as a player things are really running quite smoothly. When I try rolling stuff it immediately gives feedback. Maybe it's a GM-only thing that makes it much slower, though, so I can't really know.
7. What happened to the copper dragon mom? We want consequences to our actions, dang it!
8. I feel like the excuses for not using the Big Powerful Magical Artifacts in our quests got slimmer and slimmer, and I just went along with it instead of trying to argue about whether or not we should take e.g. the Spica Wand with us to steamroll through the dungeon. I know it's the right narrative decision, I just don't feel like the reasons given by Lissanya were enough. To compare this, in The Adventure Zone, not only were the big relics destroyed after reclaiming them (allegedly), they were also super dangerous to hold due to their mental curse powers that take over the user's personality and/or cause collateral damage.
9. The Smelling Salts are an awesome item, but sometimes I feel like they are too strong, or that you're balancing the encounters around us using them between every 1 or 2 combats. The Salts also break a ton of stuff like inability to teleport there-and-back-again in lower levels, or magical daily items, or healing. I know you don't like it when we go for an 8-hour sleep inside a dank cave, but I think the solution should be "make us more scared of sleeping in active warzones" rather than "give us an item that totally removes a central mechanic from the game".
10. Why "The Voyagers"? We aren't doing any voyages! The best you could call it is "short trips to exotic buildings followed by a return to the villa". This isn't a voyage, it's more like an errand, or a fetch quest. We should be called "The Errand Boys" or "The Go-Getters" or "The Artifact Burglars" or even something more abstract like "The Good Cause" or more to the point like "Lissanya's Useful Violent Friends That She Pays".
11. I think all of the druidic and arcane implements of the format "you have X charges. You can spend them to cast X Y Z. You regain them after long rests. Also you have +1 to [spell type] and can freely cast [something]" are boring. At least, boring relative to the other items. Sure, they are good and powerful and probably the most useful during actual gameplay, but I love it when items have more of a personality to them. For example, the Warmage's Gloves give the ability to store multiple loadouts. This is cool. The Wind Chime Earrings... are boring. The Staff of Flowers is cool (it can make flowers! And it makes daily healing berries!).
12. Finding random gemstones on the floor that are worth 50 gp is kinda lame. We already have a stable source of income - it would be more interesting and realistic if the things that we find are actual items (even if functionally we have no use for them except for selling to earn money). Like, how come we never find any useful potions, or weapons, or gear? Even if it's all shitty stuff it would be more interesting than almost literally just finding money on the floor.
13. No idea why there's a "Rarity" column in the shop document. We never care about it. If anything, there should be a "weight (lb)" column.
14. We basically never use "cover rules" in combat, which should apply lots of times, especially when one of us is casting a spell "through" allies.
15. I don't like how too many of your solutions to player abilities are too rigid. 3 times already were in a place that was just "warded against teleportation", which kinda sucks when we have fun teleport spells and items, and enemies sometimes do too. I think it would be more fun if instead of "you cannot teleport at all" it would be "your teleport spells become unstable, and each teleport has a 50% chance of sending the target to a different random location" or "The four-dimensional barriers around space are sharp and jagged in this building; each teleport deals to the teleporter 1 damage per 5 feet of teleportation distance".
16. We have very very few social encounters, which makes lots of the items and spells useless. This is probably just a result of the story, of course. And the last time we had a social encounter, Ullad just cast Zone of Truth and we skipped over the entire investigation.
17. The roleplaying or "fluff" sections feel too long to me, but I'm pretty sure this is entirely our fault for not being good roleplaying partners. And when we do roleplay it's mostly in the form of either engaging in a Q&A with your characters or slightly reacting to your plot monologues.
18. Let Ullad find some undead creatures with his Divine Sense once in a while, man :P
19. Lots of times we're waiting to come to a decision about how to progress through a trapped room. Some of us have plans (e.g. send the skeletons forward, use mage hand to trigger the sensors, fly above the ground, use invisibility...) but we keep awkwardly waiting for each other to talk or asking you questions. I think it would be more fun if we were either forced to not discuss too much ("your conversation is loud, and you hear footsteps approaching") or if we were forced to be fast (like that rolling-wall-of-fire-into-spike-pit trap). Alternatively, just pick one of us and ask them directly to tell you what they do.

Good stuff:
Basically everything else? you are just a good DM. I'm only complaining about the things that I'm not enjoying, naturally.
I am really not good at giving good criticism because I just like silently enjoying the game. But to give you examples:
You invest so much time into maps and texts and riddles and it's very nice.
The riddles are usually great (although they suffer from the "one player solves everything while the others watch" syndrome a bit too often, as is very common to riddles in RPGs). The chess battle was especially fun, surprisingly.
You are good at plots and going along with the group and hinting at us what we should do when we're stumped.
Your worldbuilding and character building are good.
The Google Doc with magical items and stuff is excellent. I know it's not perfect (you forget to update the register and the item amounts remaining), but it's still so nice to have in this way. The items itself are also of very good quality - I like the abundance of custom-made stuff and special items, even if they are mostly useless for most practical purposes. Would be smart of you to just expand this document with every campaign you make for any group of people (with local edits to make it world-appropriate, of course).
Also, you just have that ability to keep talking about stuff and carry conversations and/or monologues and I'm becoming more and more aware of how not easy it is to do.
Also, you are a cool guy and we just mesh well as a group.
Also, Xin is awesome, and you enable them (RPing, art...), and that's very awesome.

In summary, just to repeat - These are all the things that are bothering me with the game. The end result of playing, my total satisfaction, is still way up. So don't worry about these complaints, just think about them and maybe improve what you think can and should be improved!