If we were to use a different term, I’d probably go with something like “provider”, like “email provider” or “internet service provider” or “phone service provider”, you’d have a Lemmy provider. Providers tend to be associated with mostly interoperable things so it fits alright.
Anything else is a wild misuse and abuse of technical terms that have actual meanings. Discord already murdered the definition of “server” with calling their spaces “servers” banking on user ignorance and familiarity with the term to sell them on a completely different kind of service.
Bigger instances run on several servers, maybe even distributed geographically. They’re definitely not proxies. Host could work but it’s a bit nondescript. Hub doesn’t really sell the decentralization aspect. Even server doesn’t quite do it anyway, since thanks to Discord people now associate that with isolated spaces anyway.