We went with CKEditor5 with a commonmark backend and I cannot recommend it more highly. commonmark data is converted upon initially opening and when closing the editor, allowing all native functionality to happen efficiently.

However I do note that we do not yet have a production ready backend for collaborative editing. I would suggest to use their service if possible, since we're developing an open source product it would not be an option for us.

There is a quite leaning curve when implementing new plugins due to the strict separation of model and data (commonmark in our case) and user-facing views.