Integrate with menu_item_extras

I've spent about 10 hours on this in the past couple days, and I don't think it's possible with the current entity types.

I got quite far down this path -- I did manage to get the field ui showing up on individual group menus, content saving, and more -- which took patches all over the place, all in menu_item_extras.

However, the final thing needed involves loading the bundle from the menu_link_content entity -- and I'm hitting a hard blocker here. Each entity type can define a single "Bundle Entity Type." This gets stored in the entity type definitions, and looked up directly in \Drupal\Core\Entity\EntityType->getBundleEntityType() -- and I don't see any way to override this to allow either "menu" or "group_content_menu" (or anything else) -- menu_item_extras sets this to "menu" in menu_item_extras_entity_type_build(), which stores it in the entity type definition -- and there's nowhere to change it before it gets loaded.

If I remove the bundle_entity_type and add a provider instead, it doesn't load the bundle-specific field configs at all.

So this looks like a hard blocker to making this work at all -- if the menu_link_content could set a bundle_entity_type to one of the entities in this module, I think the rest would fall into place -- but you can't get there from here! At least not with that entity type.

I'm thinking the "correct" solution is to have group_content_menu module define its own menu_link_content entity -- can probably extend the original from core, and set the bundle entity type to the group_content_menu_type -- which would be far better than what I have, trying to jam it onto individual group_content_menus -- I don't see a reason the group_content_menu should be fieldable, I think it makes a lot more sense for the existing field_ui on the group_content_menu_type to apply to its menu_links, not the menu itself.

Maybe https://www.drupal.org/project/group_content_menu_bundles → can solve your problems