#[0] 's idea of using private zaps for hiding DM metadata could be generalized as using third-party trusted services to send encrypted DM blobs on your behalf.

These third-parties would be the ones interacting with the relays, not you. If the DMs leaked the public would see just that one of these servers sent a DM to someone, but they wouldn't know who was the original sender.

The receiver would get these messages and decrypt them to be able to see the actual message signed by you inside.

Relays could ban these services if they were being abusive, or charge these services directly (and then they would charge you).

Receivers could also refuse to decrypt and just discard messages from services known for being spammy.