Huh, I don't see the rebellions you are on Captain. I usually only play on Captain or below if going for an achievement, since I can tech rush ahead of the AI pretty easily with a good build so they aren't a challenge, but not seen any empires implode on themself. See the odd empire have a planet leave and usually get protectorated by a neighbouring rival, but not like the entire galaxy collapses in on itself. I'd kinda expect some empires to have rebellion issues, keeps the galaxy feeling a bit dynamic.
But I usually play low empire count + high primitives because I like that the galaxy has a mix of tech level empires at once. I've before played a huge galaxy with no other empires but 5 fallen/3 marauder and modded 25x primitives, and just...stayed in a corner with a tall empire and a few vassals and watched the galaxy do it's thing.
As for ring worlds, I find the trick is to create them in their own sector and release it as a vassal. Let them build up and then put in the effort to reintegrate them in a decade or two. Earlier I described that as gaming the system, but thinking on it I actually like the mechanic of having to leave sectors to be semi-independent and then put in work to reintegrate them later. Isn't that literally how colonies worked in real life, being left to their own devices to establish themselves then their parent empire coming in and taking control?
Vassals from sectors will grow faster in pops than sectors at the moment, so releasing sectors as vassals means only that vassals pops count towards their growth rate rather than your entire empire.
Plus the AI changes to make the AI more aggressive means that Vassals actually can help in war for a change. When I went to war with an Awakened Empire I basically didn't need to field armies because my vassal kept sending 10x Gene Warriors to invade the planets, and since I'd already took the starbase I still got the planet after they conquered it for me.
Also, thing to remember: big planets are for specialists, small planets are for resources and pops. You don't want to fully develop a small planet, you want it to be a backwater that feeds your larger and more important planets.
Rather than buffing up pop rates for empires again, I kinda think Paradox should focus more on interactions with vassals, sectors, and federations as a way of building up your powerbase. Maybe make vassals give you some resources, and automated sectors closer to micro-vassals?