To me subinstance sounds more like a technical term, but I guess people would just call them subs anyway. I think that’s a problem in general with deriving anything from “instance”.

I guess community does a good job at being a more human centric term. You have the technical side of things, servers and software (instances) and on those you have the actual user facing parts (communities) so in that way it’s kinda fitting.

Further overthinking about the terminology I just realised that Lemmy calls joining communities “subscribing” and Reddit calls it “joining”, while I would naturally think it would be more fitting the other way around.