Nice! Thanks for that idea.
Is the mobile beta editor rich enough?
Perhaps embed-able as a more general web editor?
That brings up Conclave, a real-time, collaborative text editor using WebRTC and conflict-free replicated data types.
There is also TipTap editor.
And come to think of it, WordPress devs wanted to do this with Gutenberg blocks, which has moved to the Sync Editor - using Yjs, which seems to have many implementations.
Nextcloud Text uses tiptap as well for their collaborative text editor with markdown.
Looking good with many options.