One up for this topic!
I’ve found a dozen of questions how to handle custom conditions (e.g. here, here and here).
In the case of a one-to-one-Relation it works just finde but one-to-many-results in a filtered list without content. The filtering works but the content in the collection is missing and therefore not usable for a future content editor.
Hope someone can help here!
My workaround for now:
I created some boolean fields for the belonging content type and disable them for the role I want to restrict. Then I have lifecycle methods (beforeCreate and beforeUpdate) which check if the belonging value is found in the one-to-many-relation field. My custom condition now aims to the boolean and everything just works fine.
This is just a workaround as long as the one-to-many-relation doesn’t include too many possible values (in my case 3).