Interesting you say DX is a large aspect of what you are going for. ProseMirror is awesome but the API can be somewhat complex for simple things (but at the same time incredibly powerful when you get to understand it). I absolutely love TipTap[0] which is providing a much more user friendly API on top of ProseMirror, along with a clean Plugin system.
Going to have to have a play with Lexical, excited to see Yjs support!
[0]: http://tiptap.dev