I think I will still get some benefits when adding all sources as PRIVATE. My CMAKE code will probably get easier to read. Also I am currently keeping my .CPP and .H files separately in a ‘private’ and ‘include’ folder respectively. I will have a closer look and think about it or just try out some things.
I will mark your long reply as the solution, even if @hsattler was already going in the right direction in the first reply. I just didn’t yet understand how all of this fitted together and if I actually had needed them to be public, then marking them as GENERATED within the my-app directory would have been the solution.