It's pretty easy to create @mentions by creating and inserting a custom "atom" using the function "insert_atom" to insert the atom's HTML. And then you can do further initialization by listening to "atom-will-enter".

I need to go back and turn on the ability for sections inside atoms and cards to be contentEditable=true to be able to type text directly inside the element. During development, I had disabled all mutation observation for cards and atoms so as to not corrupt undo history. But I will work to turn editability back on soon within atoms and cards - but with precision.

I had dabbled with creating a basic editor setup in ProseMirror but overall even ProseMirror itself felt a bit too opinionated for my taste. Another usability problem was that the ProseMirror example was implemented as a module instead of as a top level project. So I had started a buildless ProseMirror example but ultimately I decided to move on anyway: https://github.com/simplygreatwork/prosemirror-buildless