I'm on a journey to build a decentralized, live-programmeable and collaborative block-based editor [1] that syncs over Matrix [2], and unfortunately found that there's still not a great Open Source text editor component on par with something Notion's editor, which is why I got started with this.
BlockNote builds on top of Prosemirror [3] and TipTap [4] (two great libraries to build text editors for the web / work with ContentEditable and its quirks), but these libraries are pretty bare-bones ("headless"). While there's still some ground to cover, I hope BlockNote makes it easier to get a better out-of-the-box UX and am looking forward to your feedback!
[1] Follow along at https://www.typecell.org
[2] See this HackerNews discussion on Matrix-CRDT: https://news.ycombinator.com/item?id=29978659