CodeMirror (from the same author as ProseMirror the library under TipTap) works great on mobile, you should consider it over Monaco for that feature.
How does this compare to something like Milkdown using markdown as a native format?
I see you solved some of the hard problems (tables, etc.) with Yjs and CRDT in general.
Are the backend components also open source?
I'm not really familiar with Milkdown so I can't say.
The entirety of Vrite is open-source under AGPL-3.0 license (with parts under MIT): https://github.com/vriteio/vrite