Not to diminish your suggestion…
Have you considered using different libraries for the different “media types” (1 library for audio books, 1 library for “music”)?
That’s kind of a valid point but I’m afraid it’s not solving my problems.
My problem is not only the difference between the audio books and the music, it is more about the structure. There are folders in folders in folders, many of them don’t have album tags because I got the music from web radios or from youtube and the songs are sorted into different folders.
The data tree is complex and I would have to create at least 20 libraries for my favorite “playlists”. sometimes the songs in the folders have album tags in them and now I got this huge music library with thousands of albums and artists and the only thing I would need is a possibility to mark e.g. /music/abc/xyz/yay and /music/ccc/eee/asd/dfg/louder for sync. and when I watch my synced music library I only see those folders with some synced content in it