I looked into the Yjs implementation and first of all it is awesome! @dmonad got the CRDT technology working stable and also optimized it to avoid a big data footprint. Congratulations.
But what I think is the crucial feature is that it works serverless. No central instance you need to trust and end-to-end encryption is also doable.
Because of that I believe the technology is worth taking a closer look. Even though the existing sync mechanism works great as well, from my understanding it is more difficult to have the single steps getting applied in the right order and the whole history needs to be remembered in case an older version of the document has been used as a starting point for edits. Please correct me, if I’m wrong.
That said I agree with @marijn that it is worth using the Prosemirror State as the basis for synchronization, since this is the heart of the philosophy behind the project and why it is the best solution for rich text editing available.
This is just my personal opinion I wanted to share. Anyway all I see here is exceptional great work on all sides and would love to see development being continued.