@haubles first, big congratulations! Looking forward to seeing what the new organization can do.
Regarding the overpopulation of the flagship instances: If changing instances is made seamless-- including post migration!-- then the onboarding workflow could be:
1) someone starts on dot social;
2) they take some time to get the lay of the land and see which communities they gravitate towards;
3) they switch to another instance, now that they know where they want to be.
I think this is more beginner-friendly than asking them to choose their instance up front. Dot social could just be treated as the transition instance for new users. With this approach, I think there would be less objection to new users being steered toward dot social.
But post migration is critical for this to work.