Support for CKEditor 5 (e.g. embed buttons)

I think you need to take a look at the entity_embed module in combination with entity_browser

I created a separate module which integrates entity_browser with ckeditor5:
https://www.drupal.org/project/ckeditor5_entity_browser →

It works on the link UI level, so you open a modal for a configured entity browser and insert the link in the link UI. Siince embedding entire entities is something else.

FYI the Embed and Entity Embed modules fully support CKEditor5 now in their latest stable releases. We also have test coverage running for both CKEditor4 and CKEditor5 in the module without any changes and both are passing.