There's a more fundamental question here I think. I can understand wanting to get an older, theoretically unsupported app running on Catalina, but that makes less sense re: iTunes as AFAIK no functionality was lost between Mojave and Catalina. iTunes got renamed to Music, and the syncing functionality was moved into Finder. Is the integration between those two functions _that_ critical?
If you use audiobooks then Catalina is a total disaster. You lost smart playlists, the ability edit ID3 tags, the sync to my iPod doesn’t work well anymore. Its really bad.
Keep in mind that iTunes and Audible audiobooks are DRM'd, so you can't necessarily just mark it as a different type of file. (The DRM is crackable, but then that's an extra step.)