Ya what the ****. I had to do the same restoring process when I got a new MacBook. Backing up to iCloud or iTunes works for everything but music? why? So I started using an app that comes with Setapp called AnyTrans which is an alternative iDevice manager that actually let me back up my music, BUT NOT playlists. I realize the backing up of music/playlists has never been an option through iTunes but since when has simply updating to a new OS on a MacBook that's already been synced with the same iPhone called for having to wipe the media from it? I, along with most others I assume, use playlists to be able to listen to a particular set of songs or albums without having to search through your library and remember which songs/artists/albums were recently added or added b/w certain time frames. whatever the reason, i dont remember exactly what songs were on my iphone previously and even if i did, going through and adding just the ones you want any time you upgrade your phone or macbook or iOS or OS is nonsensical and unnecessary.
Lastly, half of the time when using the search feature in Music, something i used to do regularly in iTunes sometimes wont even result in finding a song that I know is in my library. I know its in my library because after searching fails, going back to the songs menu and sorting by artist or song name and scrolling through thousands of songs I'm able to find it. and ive had plenty of attempts doing this to make sure i spelled correctly in the search bar. for all the benefits of catalina i cant say theyre necessarily worth losing the simple ability of managing music on your device, especially not for locating music on your own computer. if going back to mojave wasn't such a long process i'd do it. for anyone that hasnt updated to catalina, if your music is one of the main things you use, do, listen to, whatever, I'd highly suggest not updating. I read the same thing and figured how bad could it be. hindsights always 20/20. sorry for the long winded post but had to vent the aggravation this has caused