Civics: Inward Perfection + Agrarian Idyll
What did you take for your third civic?
I'm in the middle of a very similar run. However, I'm pacifist/materialist/xenophobe. I started with Inwards Perfection and Technocracy and picked up Agrarian Idyll later on. Technocracy is absolutely nuts in combination with these other two. Inward Perfection gives you a unity boost, Technocracy gives you a solid chunk of science for every colony plus unity from scientists, and pacifism gives you a bit of admin cap. With all this, you can actually expand pretty far without really falling far behind tech- or tradition-wise. Getting Agrarian Idyll late means a bit of refactoring to get rid of city districts, but to me, the Technocracy opener is stronger, since until that mid-game tech, you only get one extra housing from Idyll farms, not two.
Materialist also means I go for robots, and with that extra population growth, by the time I built my first ringworld segment, I had 1-4 unemployed pops from 8 of my 10 colonies clamoring to move in. I was more limited by building speed than I was by pop growth, especially since I'd just recently upgraded to Erudite/Fertile. It was one of the first times I said "Whelp, time to pause my tradition growth so I can pick up Architectural Renaissance" and it was for the building speed as much as the megastructure speed.
I kept my urban development even smaller than you describe. I eliminated city districts mid-game, except for one trade planet that had like 8 rural district slots on a size 18 world, though I do have them on the ringworld. Instead, I expanded my alloy/consumer goods production on my habitats. With just three that were growing pretty slowly, I was able to keep up with alloys and consumer in great shape; I never had to wait for alloys when building my ringworld. With 6 housing/2 entertainer, you can get up to the 70 needed for a full set of buildings quite easily, and as you upgrade them, you can change some of those buildings over to rare resource refineries if you have more jobs than space.
I'm also pretty rapidly moving all pops from my other worlds to my ringworld. My goal is to reduce every other colony down to a bare-bones mining colony with mining districts plus the few buildings they support. This also leaves these colonies almost entirely robots who can specialize in mining, while my erudite main species moves to research/farming/generator ringworlds.
This last bit of moving most everyone to ringworlds (since I have a second on the way already) is more of an RP thing and experiment rather than something I know is super valuable. But it's 2375 and I'm doing great with minerals with only a level 1 Matter Decompressor. I'm already working on the second ringworld while the first fills up, so things are looking good as I move into end game.
I got my first ringworld segment colonized in 2345 and completed the whole thing by 2375. Around what time did you get your first ringworld segment built and when did you get it completed?