I'm running haven and currently use that for inbox, is that good for this? Otherwise my public relay is nostr-rs-relay.

I don't think it deletes NIP17 DMs. But it is your server, you can just go there and delete whatever you want :)

Now when You say "Better than Signal"... Infrastructure and centralization aside, are You saying this is actually more secure than Signal? I mean Signal the mobile app, not the desktop version.