> a problem I have now in Amethyst, which is that many notes are only "seen" by one or two relays

This should not be a problem, it is how the Outbox model works. Any other additional relay comes from a broadcast from users that interact with the event and this organically increase the decentralization and reachability of user's content.

It seems that you want to create an hybrid client/really, that actually could be useful in edge cases to support "thin clients", especially on mobile.