NIP-29 is designed to allow groups to move to other relays. That is the entire point of it. There are no options besides that that aren't horribly inefficient.
And that's why I was always against relays doing much stuff especially around keeping stuff secret. Keep relays dumb and replaceable. Let anybody (who pays for the traffic) pull a copy.
Private groups that communicate only via a specific relay are not censorship resistant if the relay pulls the plug because of some lunatics in London.
nostr:npub180cvv07tjdrrgpa0j7j7tmnyl2yr6yr7l8j4s3evf6u64th6gkwsyjh6w6