You didn't ask for advice, so feel free not to take it - but you should consider using a pre-baked editing solution like Quill.js or something similar. Rich text serialization is its own problem and you shouldn't waste your development time on it :)
As someone who was experimenting with text editing in a browser recently: I could not find any open source (non-commercial expensive license) text editors that could actually handle workloads I cared about. (and which are not also specialized on code, such as Ace Editor) While Quill (and friends) definitely provide some very nice functionality, the moment I tried to do something like shove a full novel into one, not only did it take quite a while to actually load that text, but it took 500ms+ to insert single characters. There is a serious lack of an open source text editing library for Javascript that can handle any amount of characters without completely bogging down.
I was actually seriously impressed that I was able to take a huge amount of text and shove it into Obsidian and the UI stayed completely smooth the entire time. Props to the Obsidian devs for that!
