One way (which I know not everyone agrees with me on) is to have the Nostr equivalent of Mastodon's "instance federation" system.

In other words, for all relays who chose to do so, they would sync with every other known relay using relay software that supports that, like strfry.

This way, no matter which relay a user connects to, as long as that relay uses this functionality, they'll have good coverage of the posts of the users they follow.

If most relays do this, then it'll be possible for clients to intelligently pick which relays they connect to based on evenly distributing the load across relays, and they won't have to worry about missing out on events.