I think we should really look into a Dart based implementation of the Signal protocol. Afaik Mr. Kaine proposed an off-chain messaging layer based on it. Mixin desktop app uses it and I also found this social media app using it. I’ll need to check what Mr. Kaine was proposing a while back. I see that there is a Rust based implementation that’s actively maintained by Signal.
Also I don’t think on-chain messages are a good idea right now.