This is something that needs to happen more - The whole point of the fediverse is that you don't need any high population instances. Look at the situation with lemmy.ml - they're hitting major infra issues as a result of their high user count, but they're still accepting new users (as far as I can tell). Just close the doors and post a list of reccomended other instances with similar focuses.
You can still access all the same content, regardless of your instance, or even platform