There already is a crowding penalty for planets near the population cap.
In older versions, I used to be able to sign trade charters with splinter colonies, but I'm no longer seeing that option. Was it intended that you could trade with splinter colonies, or no? I thought it made sense to swap goods with tiny enclaves of Humans and Ashdar, and missed being able to trade with them since that changed.
I think that would be a nice perk and a slight excuse not to conquer everyone, to be able to sign trading agreements with splinter colonies. It would have to be appropriately scaled, perhaps no more than 1 route.