The Signal protocol is the best what you can get for private and secure DMs at least for now.

The only thing I don’t like about them is that they still insist on a phone number.

Don’t get me wrong. I 💜 nostr, but current client implementation of DMs are not well suited for high sensible data. It’s a killer for digital censorship and a promising contender to ensure free speech in the digital realm and that’s exactly what we need ✊💜