Mirroring n-to-n relays not sustainable because the traffic grows exponentially. I think the right approach would be new relays seeding off of existing relays, and optionally maintaining their own mirrors. Again this is not to shift the responsibility of the clients being the ones connecting to multiple relays.
Discussion
personally I don't think relays should be mirroring if mass adoption occurs. they only way nostr survives censorship is if users understand vaguely the mechanics of nostr and take responsibility of their own mirroring. otherwise it will be hard to navigate when relays go down.
Iāve been working on compatibility with hypercore crypto. Your nostr identity can be a hypercore and it can be queried p2p - as an alternative to only relays.
A few hurdles so far. Iāll try work on this again this weekend.