We did look into editor.js and the API seems not as advanced as Slate's, mostly because Slate tries to replicate the DOM's behavior with Nodes and Ranges, so you can make edits in a very fine grained way.

If you're going for a new text editor, I'd start with Tiptap and try to check how to do advanced custom stuff with Prosemirror. Maybe if Tiptap 2 was out a year ago, we would have started with that.

For collaborative editing, definitely check out https://yjs.dev