With my huge iTunes library and satisfaction with Mojave doing everything i need on my iMac, this is another killer reason to ignore the Catalina catastrophe.
I also came from High Sierra and also didn't want to update for a long time, but at some point I felt like I want to do it and now I am glad that I did it, although I haven't gathered any long term experience with Catalina... Since I were installing from scratch, I didn't want to settle on an "old" OS *-)
I fixed the separated Library Issue by re-dragging the iTunes Media Folder (seen on the screenshot above) into the music app (Sidebar on the left Side).
This caused the Library to double - so the files from iTunes Media and the same exact copy of those Files were now in the music Media Folder.
I checked the Paths in music.app and they all referred to the new Library, which lead me to just erase the old iTunes Library - and surprise - it all seems to be working perfectly fine. The Date when the files were added also is still the original from the first date of adding the files...
BIG Thank you! to
darngooddesign his post actually made me wonder what re-adding the library would do.