The way it probably _should_ work is everyone has a large number of "app relays." Those store your profile data so you can move seamlessly between clients.
Everyone should have a smaller number of inbox relays, probably mostly topical or community-based, but anyone who wants to @ you would just post to one of your inbox relays and you'd see it immediately.
Lastly, I could imagine needing only one outbox relay. Your profile lists the outbox, so people know where to find your posts.
In a follow-based feed, the client should be smart enough to query different relays based on your follows' outbox lists.