Couldn't DMs be encrypted by two keys? In case your main nsec gets compromised you aren't completely fucked? Would increase complexity though
Discussion
Use ur own relay, anyway a must.
Great for talking to myself
And for storing in a hidden sidechannel the DM unencrypted or encrypted by a second key in case u lost ur first, OTOH u could also instant repost any DM encrypted with the pub of a second seckey or later in bulk from ur relay .
Clients would only need to store all keys per social id they accept trusted too to see again ur old DM if they switch to ur new keypub.
Might need a nip-xx but not necessary.
A 2-of-3 multisig would be interesting. You control one key on the phone, a second key by a trusted bot (running on your node at home and auto-signing events based on rules), and a 3rd key offline as backup.
The only trick then is that the "multisig pubkey" is the actual profile pubkey.
Don't see a reason why it wouldn't work.