So I'm finally okay with my situation. I deleted the new Music library from ~/Music/Music
Then noticed that in ~/Music/iTunes, my original iTunes library files from before the upgrade are still present.
After deleting the Music library, I opened up the Music app while holding down the Option key to get the 'Choose Music Library' screen and chose the old iTunes library. It reprocessed everything, and MOST of the album artwork started to populate.
There was still a fair bit missing, so I deleted the Music library again, restored the latest iTunes Library from before the Catalina upgrade, and tried it again. This time, it loaded about half of the artwork. So I deleted and tried again, and then it loaded almost no artwork at all!
I rebooted, deleted the Music library one more time, and left the iTunes folder as it was before I restored any backups. So it was now in the exact state it would have been after the upgrade. I opened music holding Option, picked the iTunes library, and let it process. Once again, MOST of the artwork was there. It seems to me like most of what was missing was artwork I had previously had iTunes fetch from the store. I then spent a few hours fixing the small percentage that was still missing, but at least now it was manageable. Before, it would have taken me more time than I practically have in order to fix everything.
Another tip: You can find a most (at least in my case) of the old iTunes album artwork in ~/Music/iTunes/Album Artwork but it will not be in a format you can use. I ran this folder through File Juicer by Echo One, and it extracted all the covers in JPG and PNG format. You can run the folder through the free version just to see if this will help you and purchase it if you think it will. (Not affiliated with them in any way, just REALLY helped me to save the rarer artwork that I had trouble finding online.)
Keep in mind, any artwork you recover in this manner will have a random filename and you will need to match it back to the album just by looking for it visually.
So anyway, my recommendation, based purely on my experience with this whole thing:
- Rename ~/Music/Music to ~/Music/Music.bak
- Reboot
- Open Music while holding down the Option key
- Pick 'Choose Library' and browse to your original iTunes library in ~/Music/iTunes or whatever alternate location you may have placed it in
- It will re-convert your iTunes library to a Music library, and should reprocess all of the artwork. Don't close Music until this is finished, as indicated in the bottom left corner of the application.
- Hopefully at this point you will have most of your artwork back without the need to restore any backups of your entire music library or your entire system.
Following steps are optional, but may be helpful in case you are still missing a great deal of artwork after this:
- Download File Juicer by Echo One
- Drag the entire ~/Music/iTunes/Album Artwork folder over File Juicer. It will place a folder on your desktop with all of the artwork images
- Visually look through the folder and find the artwork you are needing. Copy/paste or drag/drop this back into your album in the Music app.