Okay, after some testing, I want to share my experience. I found three programs, that are cost-free and meet my needs. These are Data Crow, Libra and GCStar. Data Crow has the most options and functionality, but it lacks the classic shelf look and has some bugs. Also I experienced one serious bug, when I deleted a whole module/library, that should not be deletable at all.
Libra on the other hand has less options and is less configurable, but is rather stable. It has the mentioned shelf look, though it looks a bit amateurish.
The last one is GCStar. It looks and feels like a professional programm. It has a variety of skins to choose like wood shelves, glass etc. I experienced some minor bugs, but nothing drastic, so I stick with it to build my offline GOG-library.
Maybe some of you want to check it out.

And no, I am not involved in GCStar development :)

If there was a way to modify CDisplayEX and MovieMonkey then it would be nice.
Btw I think GOG galaxy can be modified in that way.