It's hard to make levies drop to zero because modifiers exist and they are all added up, you can't just throw -25% levies and expect it to be 25% less levies. Dropping them all the way to 0 also makes buildings with levies involved that much more useless, so idk. Reducing it is a good idea imo, something like -10% each era will make lategame armies less ridiculously massive and would help the AI coz it raises all for every war..I would make it so as you go up the tech ladder you get more development, demesne, and MAA, but you get less levies (to the point you get zero in endgame), vassal taxes (also trending towards zero), negative vassal opinion modifiers that get harder and harder to surmount, and decreased vassal limit. This is to simulate centralization as the middle ages come to a close.
LESS vassal taxes and LESS vassal opinion? That doesnt sound like centralization... And im not sure how AI deals with being over vassal limit, i doubt that happens very often..
Everything else is already a part of the tech tree, dev growth bonuses should be bigger probably, and there could be domain limit bonus in every era, not jus the first and the last one.. MAAs idk about, vanilla gives a lot of them out, i even reduced that somewhat.
As an aside, i think some of those would be better suited for high-absolute crown authorities instead, as that's something vassals can push to revert and would make it a bit more granular.