Your approach looks promising. One question: how do you handle conflict resolution at scale? ProseMirror's steps model helps, but once you mix in multiple cursors, mobile clients, and permission controls, things get complicated fast. If you've built extra abstractions or a custom syncing layer on top of ProseMirror, I'd be very interested in how you approached it.
Overall, impressive work - it’s refreshing to see an editor that aims for minimalism without sacrificing collaboration features. Bookmarking this to test on a few writing workflows.