And lastly before I go to bed, my ideas of the mixed policies and the balancing.

The balance of the mixed policies are a bit all over the place, but I like that, but I think the more powerful ones should be with the more unusual policies combinations, so that you can motivate going for Rationalism and Militarism if they have a really powerful Mixed Policy.
I'm also not certain if you need to complete all of Rationalism and Pacifism in order to choose Liberalism, for example. Or if you only need to have chosen those main policies. But I will take it as if these are for mid-late game.

I'll go thru them one by one.

Wonder Builder - It's kind of underwhelming, a Worker/Engineer is pretty taxing to build due to them costing a citizen. So 600 production would only ever get used in the very very early game if you are worried civs are about to beat you to a wonder. But in order to utilize that you'd have to go pretty hard on culture and gold in order to recruit one.. So you'd lose more time than you'd gain. I'd suggest instead like a "Workers can be converted to +100% production in the given city for X minutes" or something. That would scale well with the entire game.

World Congress - Yeah, it's some influence. Nice. It's a common combination of main policies, not really overwhelming but not underwhelming either.

World Health Organization - I've spoken about this before, but I find it very interesting that with it you essentially don't have a cap on the number of citizen you can have, and you have an active way to combat low health. Powerful Policy, with unusual combination of main policies. +1

Police State - Militarism isn't that common, Imperialism is though, but I think the effect is very underwhelming anyways. +10% happiness, at the point of you being able to build a courthouse you should essentially have more than 100%+ happiness from colonies anyways. Most cases, this policy does nothing.

Platonic Academy - I like this, more trade routes are really nice. But I think the +5 science / trade route doesn't do a whole lot, unless you go for Horus you won't really have more than 10ish trade routes when you're late game. and another 50 science/s won't really make a difference. I can only see it being useful very early game.

Square of Miracles - Good, another spell cooldown reduction, they are handy.

Organized Slavery - This one sounds interesting, I haven't tried slave attacks to know how effective they will be, perhaps if they are really common Wonder Building policy will be decent too.

Crusades - We've talked about this. 10 is better than 1 :)

Proselytism - I love this one. I'd probably say that this is one of the more powerful ones, but I also think it's great. a percentage scales really well in the later on. For my save, having another almost 90% faith really helps me farm out those anhks for a good reincarnation. If anything, perhaps it shouldn't require Imperialism and Piety, as those are both common main policies.

Liberalism - Like this, from what I gather building Progress will be a good way to earn your Industrial points, and buffing that is great.

National Institute of Health - Basically lets you to get 1 more citizen in every city. It's not super good, but it's not bad either. I'd suggest changing the requirement for this one for one of the more common main policy combinations.

Civilizing Mission - I like this one. But how powerful it is differs depending on how it would work- if it works only when you take over a city then it's alright, it's a fine bonus to Industrial Points. But if it also includes when you burn cities - Which I personally think it should - Then it'll give you a good reason to build a army and going around burning cities as an alternative way of getting industrial points. Combination of Main policies are on point though.