and finally, what if we had invite-only/private communities?
We do have those, you can have instances not federate and be invite only. But lets face it discord does that better.
I think that together these are pretty reasonable and would satisfy OP.
None of those are reasonable and most break the very core concept of federation. What you are proposing is to burn down the fediverse in order to protect groups who are not asking for this.
Because that is how people learn.