I really like Slate....but the ability to do collaborative editing is relatively new.

Have you done anything with collaborative editing with Slate? If so, how are you finding it?

Collaboration isn't a requirement for me. I would definitely go with quill instead if it were. They put a lot of thought into making the delta format natural for operational transforms: https://quilljs.com/docs/delta/

Your document state is always represented by a list of deltas. Two people trying to change the same document would just have their individual list of ops "rebased" together using OT before being concatenated into the existing list of ops, forming a new document that is still just a list of ops.

I second using quill. It's really modular and the code itself is so clean.

Quill has a great API, it seems like they really thought things through and designed it (what's a good word for the opposite of ad-hoc?). Even the lower-level bits are available with a clean API if you should ever need them (e.g. it's super simple to use the underlying OT and diff/merge functions on whatever text you want)