Did you consider Tiptap/Hocuspocus (also built atop ProseMirror/Y.js)? If yes, what were they lacking that lead you to roll your own? In any case, thanks for the writeup, it's timely to see someone work through this as I tinker with a collaborative editing project of my own.
I did consider tiptap/hocuspocus, but hocuspocus costs more than I can spend on a personal project right now. Additionally, the architecture of the app I am building makes heavy use of subdocuments, something that most of the yjs ecosystem doesn't support. I am likely going to end up using tiptap but swapping out the stable version of y-prosemirror it depends on with my fork that has what is needed to suport subdocuments. I really do love what the team at überdosis are doing though, I am really excited to see what kind of design choices hocuspocus makes behind the scenes, since my current server is kind of esoteric, and I'm sure they will have come up with a much better way to do it.