one of the achievements of lemmy might be as a nice platform for simply running a forum for whatever community you want all without needing to worry about federation.

I’m interested in this question for the inverse reason: being able to run a federated community on a Lemmy server which is not open-invite

People in the Lemmyverse would be able to use the community as normal, but running the community on its own server would not involve opening the door to registration by randos on that server.