Open a modal for the Add / Edit entity forms

+1 for a modal...

This has my +1 as well

Yeah this would make a lot of sense!

We could try it simply this way: https://mark.ie/blog/very-basic-example-of-editing-drupal-nodes-in-a-modal/
Or take a look at similar modules and how they solve it!

I guess the most problematic part will be to only close the modal, if the entity was saved successfully (without errors) or we have to accept imperfect UX.

https://www.drupal.org/project/edit_media_modal → might give us examples how to implement the modal!