Backpressure is what you get when a source Observable is emitting items faster than a Subscriber can consume them. It’s most often a concern with hot observables, not cold ones like your network requests.
I think you should use Completable instead of Observable<Void> in your saveUser method, and use Single for all places where you follow a request/response or input/output pattern. Observable should be used when you actually want a continuous stream of events.