I had the same problem, and I didn't want to manually download and embed artwork for every album (too many albums in my library). Even with the Metadatics app it was way too manual to select artwork for my entire library.
So I wrote a Python script to batch-download the artwork from Apple Music and embed the artwork directly into my audio files (m4a and mp3s). Hopefully this will help someone else as well.
https://github.com/regosen/get_cover_art
https://pypi.org/project/get-cover-art/
NOTE: Once I ran the script and got all the artwork embedded, I still had to export/re-import my library like this:
- File->Library->Export Library... and name your exported library file.
- Visit Music->Preferences...->Files and screenshot your options. You'll need to restore them later.
- Quit the app and relaunch while holding down the Option key.
- Choose "Create Library..." and pick a new location.
- Visit Music->Preferences...->Files and restore your desired options.
- File->Library->Import Playlist... and choose your library file from step 1. (Yes, it's called "Import Playlist..." but you actually use this to import your library.)