Memmy is really good. The TestFlight is full but it’s about to get released on the App Store
wefwef is a web app and it’s also really good. You can install it to your home screen and you’d never know it wasn’t a native app.
Mlem has potential but so far it’s got a bit of work to do to catch up to the above two.
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.