I've been thinking of a couple ideas for changes.
- I'm annoyed at how few leaders we can have, it feels like I always try to skirt by with 1-2 admirals and 1 general, and I don't have enough administrators for even the original cap of 5 worlds plus the sectors.
Frankly I'm not even sure why we are so strictly limited.
So I'd like to buff the base leaders, maybe to 20. Possibly make the techs give more each.
- High level leaders are too powerful, long-lived species are already much better than others for keeping their good leaders, now they also get to have them become much better.
So I think I'd prefer reducing it to 5 again, that also gets around the graphical error where it won't display how many levels you have. It's also a problem when Assist Research can give twice as much bonus as it was originally meant for.
If we want leaders to matter more then lets have events allow them to develop traits and make the traits matter more. We can also increase the benefit each skill level and slow down the progress but I think the old 5 limit is fine unless we plan to adjust a bunch of other stuff as well.
The rest are related to ethics.
I'm a bit unsure why you reduced the materialism tech bonus, did you feel they tended to get too far ahead or did you just want to slow down overall tech speed? Because if it's the latter I'd rather the techs just all cost more points.
Spiritualism vs Materialism
I feel spiritualism is much stronger than materialism. You get special techs and the happiness bonus translates to a direct bonus to everything including research on most of your worlds. So ironically they often perform better at research than the materialists..
Also they get a rather overpowered government for enslavements. I'd be tempted to reduce the Transcendent Empire to 90% slavery tolerance. That way there'd be at least some unhappiness among non-collectivists.
In terms of lore, being a materialist means you are willing to pursue any research opportunities by any means regardless of most ethical/moral limitations. They need to have more advantage from this, I'm tempted to double their tech bonus to 10/20 but I guess that could be too powerful. Then again I'm not sure, other factors can already get it quite high and currently materialists don't actually do research all that well.
Edit: I'll just go with 5/15 I think.
In terms of lore, being a spiritualist means intentionally barring yourself from certain methods or paths based on dogma or out of concerns for whatever is spiritually correct. This would justify a research malus, at the very least enough to cover the bonus they already got from happiness.
Beyond that I'd suggest making their psionics tech-line non-exclusive and instead just let them have early access.
The way to do that would be include a new physics tech placed late game, after completion you qualify for getting the Psionics Theory tech. It makes sense that non-spiritualists can get the tech as there actually IS very reliable evidence that the phenomenon is quite real in the Stellaris setting, researching the physics tech allows you to partially understand how it might work scientifically as opposed to being seen as some kind of magic. Also Materialist Fallen Empires have psionics, just saying..
Collectivism vs Individualism vs Xenophobia
First things first, collectivism is plain overpowered. It has everything xenophobia has just with less disadvantages and more extra fluff. Individualism isn't all that bad really, it's just hard to compete with collectivism.
Xenophobia's alien slavery tolerance doesn't stack with regular slavery tolerance for some reason, so unless you are fanatically xenophobic forget about implementing alien slavery. Meanwhile a spiritual collectivist does fine with the regular version and isn't limited to aliens, he can enslave his own people without objections.
A collectivist regime can even legalize Purges (Sanctioning genocide of billions of citizens) without unhappiness. I think that that particular policy should generate unhappiness regardless, perhaps reduced for the fanatics. For Xenophobes the "xenophobes only" option should be better than the universal option as it appeals to the Us-vs-Them line of thinking, so their own citizens shouldn't be unhappy at the policy, other races should though. Then again maybe only fanatics should be completely fine with it.
In terms of lore, a collectivist regime is quite totalitarian, even if potentially well-intentioned. Clearly free speech is not a given. In light of that it seems appropriate to me that they get a disadvantage in tech from all the suppression they are doing. Also all pops who are NOT a collectivist should be somewhat unhappy about the regime.
Individualists are already a fairly worthy ethic though you are also signing away any ability to do slavery. I'd give them one decent buff, a bonus to happiness that only applies to individualist pops who are ruled by an individualist government. It wouldn't make sense if they were happy by themselves, they should be happy about all the rights and freedom they are getting. This will also serve to indirectly increase tech.
To improve Xenophobia I'd suggest improving the governments Despotic Empire and Martial Empire by giving them 50% alien slavery tolerance. That way provided your pops are at least regular xenophobes they'll be ok with any amount of slavery.
Edit: I think it should be 25% for despotic and 50% for the upgraded version.
Beyond that xenophobic regimes are fairly harsh so maybe they should qualify for Frontier Commissars? Or should that remain collectivist exclusive?
Militarism vs Pacifism
I think all pops should be given 10% unhappiness whenever you are at offensive war. Then militarist pops should get 5%/10% bonus to happiness (what they already have) which mitigates the penalty. The thing is I don't think it makes sense for a militarist to be outright happy at being at war, at most a fanatic might be indifferent about it. This way they are only given a real happiness bonus if fighting a defensive war, but that actually makes sense as that would provoke patriotism.
For pacifists they should get an extra 5/10% unhappiness for being part of an offensive war. Buff-wise they should get a happiness bonus for being at peace. I haven't really played pacifists so I'm not sure how balanced they are.