Global overview for all entity type's form displays & field displays

That sounds like it could be useful.

> In a later step it would be even better to be able to edit them on this central page, but I think that will not be simple-

It would be fairly easy to make a link to the relevant 'Manage display' page, and append a redirect destination to when the user saves that form they come back to this central list.

I'm working on something similar: given a field storage, I'd like to mark one field instance's display configuration as "canonical", and then ask the system to give me an overview of diffs for all other instances of the field.

I'd then like to be able to mark particular parts of the non-canonical configuration to be overwritten. For example, I might not care about a different description in some cases, but I do care about the type of form widget that is used. In other cases, I might want to enforce the descriptions to be the same. I'm thinking of some checkboxes next to each key in the settings array.