Allow reference to all entity types

Made a proof of concept, that adds an All items checkbox, I've not done any thorough testing, and not even sure it's the right approach, but it works in my simple use case.

You can achieve this by using the 'exclude' option. I.e. rather than 'including', exclude.
This was one of the original use cases for this module, which was developed when I wrote Entity Pilot.
You can see it in action in Entity Pilot module here

The problem with using "exclude" and not selecting anything is I need to go over all the allowed bundles and check them before I can save the field instance.

Maybe the fix would be making the bundle selection optional.

Hey, I'm not sure what you're saying - is there a bug in the UI that prevents you from saving or something?
Can you provide a screenshot/screenrecording to explain further?

I'm prevented from saving, because I haven't selected which bundles is allowed for each of the entity types I want to reference.

The screenshot if from a clean Drupal install on simplytest.me running DER 3.2.1

Ah, then that is a bug.