If I'm not mistaken DM's are already encrypted, but all relays can see which npubs are DMing other npubs. That part it clear, unencrypted data

There are methods (like what I mentioned above) which improve private DM's (NIP-96, Outbox model, etc.) But main issue is still as a relay operator, I can see who you are DMing, at what time, what size of the contents in that message, etc.

Just can't see actual content.