Many admins say it is a lot of work and they put a lot of effort, money and energy in it. Some even close their instance after few years, because it is just too much.

I would say you should limit the number of active accounts. Probably to a few hundred at most in the beginning. Close registration when things start to get to become too much. Re-evaluate and re-open as necessary. There’s no reason you have to allow thousands of people on your instance.