I’m also really interested in this!
Using BSTW for generating custom widgets for SO Layout Builder is the right way to go (since SO LB uses BSTW as default), but is fairly hard to achieve when you need more than 1 TinyMCE.
In case you only need 1 TinyMCE (in Layout Builder), try to extend the existing WP_Widget_Black_Studio_TinyMCE class. I did so with success here.
