This repo is pretty easy to use. I say that as a ruby fanboy though. https://github.com/dtonon/nostr-ruby

In your gitlab you have this communications happening over DM. Are you expecting this to be a group chat? I ask because I'm thinking about how users who are trying to join would discover each other. I'm also trying to think about how you control for user input since even though you'd expect only coinjoining messages to appear here, you could just as easily get user input instead, even user input designed to exploit the code which is trying to batch transactions together.