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.