Okay, that was a six hour reading adventure. Can you entirely replace the Shadowrun rules with Stars Without Number + Supplements without losing the heart of what makes SR feel like SR?

Yes.

Guns and grenades and katanas and nanowires (future weapons) - Almost purely descriptive, aside from things like "grenade launchers shoot an explosion out to X range". Let's just...ignore chunky salsa. Or better yet, just say "if you crit fail your reflex save against a grenade, it deals double damage" and move on. The relatively brief weapon and gear tables in SWN cover this adequately, with further differentiation coming from small changes to ranges/ammo capacities/mod installations and a random table of manufacturers and model names. Shiawase Arms "Hyperion" MK IV Linear Accelerator Projectile System.

High risk combat - Strictly mechanical. A function of HP values, attack accuracy, and weapon/spell/effect damage values. Thin margins make combat more dangerous. If baseline SWN doesn't offer this, add an extra die of damage to every weapon. /shrug SR5 claimed that it worked this way, but it was actually incredibly difficult to deal meaningful damage to anyone wearing good armor. SR also splits damage between lethal and stun, but "nonlethal damage" is a concept that exists in d20-style games, so we can still have our Stunballs and Stick-N-Shock rounds.

Magic (spells and summoning) - SWN has psionics, which are a very particular flavor of supernatural, but also has rules at the end for Space Magic; Arcanists (generalist casters), Magisters (specialists), and Adepts (hyperspecialized/intrinsic powers, like warlocks or monks). Also, SWN has "Codex of the Black Sun", an entire book covering a more dedicated approach to magic that leans harder on not making wizards the best at everything while also covering spells, summoning, and even enchanted items/arcanotech in an interesting way.

Core Archetypes
- Street Samurai/PhysAdept/MystAdept: You can get there through Warriors + Cyberware; the Adept class options and some of the stuff from Codex of the Black Sun cover the other Phys/MystAd types.
- Infiltrator/Face: Expert + focus options + gear.
- Decker/Technomancer - Core hacking rules are perfectly functional for this, including on-site requirements, although no full-immersion VR options. Which, honestly, is fine. The whole hacking system is streamlined down to the same pacing/resolution as everything else.
- Mage/Shaman - Psionics (but you'd have to ditch the Teleportation school); a tremendous number of options from Codex of the Black Sun. Spirit summoning comes in under the "Pacter" class and would end up being more like the strict Mage/Shaman divide in pre-SR5 editions (one gets only spells, one gets only summoning). Even includes options to replicate Limited Spellcasting.
- Weird crap, like a PhysAd with all their points in Hacking: Surprisingly yes, if you take, like... Arcane Expert as a class and then pick up one of the focus options that gives you the thing you want.

Hacking - Core rules cover it, "Polychrome" expansion offers more details beyond subverting local security systems, including creating identities, stealing money (it creates temporary cash called "joss" that vanishes after a set time!), acquiring information from various sources, and changing existing records. But seriously, it's one skill check against a difficulty modified by location/circumstance/speed-of-execution. Bless Kevin Crawford's heart.

Body augmentation - Extensive cyberware options, including rules for System Shock that limit pieces installed and make it gradually harder to be healed through magic or biotech if you have a shitload of metal in you.

Drones and robots - Plenty of options in the core rules AND you can be a competent rigger, including all the lonely fun of mathing out drone hardpoint customizations. Playing a rigger medic is entirely plausible and doesn't require a PhD in symbolic logic.

Alt-future Earth setting - Corporations, Weird SR setting stuff like Technomancers; all pure fluff. Port 'em all over from SR5.
"Fantasy" metatypes - Elves, dwarves, orcs, trolls, and I suppose all the weirder niche crap in splatbooks) - Character creation is already pretty simple; it wouldn't be hard to balance "roll this stat twice and take the better result" against "roll twice and take the worse result" for racial effects.

Big-deal setting threats - Great Dragons, Knight Errant, Eagle Warriors, MCT Zero-Zones - Core rules include a bestiary covering broad archetypes of foes:
 - Rent-a-cop: HD 1 (4 hp), AC 10 (unarmored), Atk +1, Dmg By Weapon (semi-auto pistol 1d6), Move 10m, Morale Level 8, Skills +1, Saves 15+
 - Terrifying Apex Predator: HD 8 (32 HP), AC 16, Atk +8 x 2, Dmg 1d10 each, Move 20m, Morale Level 9, Skills +2, Saves 11+