Yeah – I haven't thought through the implementation (and it's obviously limited by how much you want to trust the infra provider where the relay lives) but it's a fun idea.
Discussion
True, but if encrypted, shouldn't be able to decrypt.
The hardest part would be client integration. I can easily right a script to spin up new relays with random endpoint.
But configuring a client to allow setting PMs to using only the revolving relays would be troublesome.
That said, you could probably bake in all the rotating relays behind a single proxy endpoint like with this https://github.com/Dolu89/nostr-proxy
Write* lol