I have it set up so that a bot user is subscribing, not my user. That way the communities are populated with comments and appear in All if I’m interested in the future, but otherwise I wouldn’t see them. It makes my instance feel much more alive.
Also, it’s worth noting that you can provide a list of the Lemmy instances in the config. You can be as obscure as you like here.
I think the job of seeking out smaller, more niche communities has to be a manual one.
The problem is that you can’t have spaces or capitals. The error message isn’t very useful, it tells you to match the requested format but doesn’t tell you the format that’s being requested.