He'll forgive me for not name-checking him here, but someone posted this (below) elsewhere, and I tried it and it works!! It's still a pain having to remove tracks (Finder/iPod Classic/Manage Storage) individually, but I was able to add tracks to my iPod successfully:
How to add music from OSX Catalina to iPod Classic without iTunes.
1. Add new music to Apple’s “Music” app on Mac.
2. Close “Music” app.
3. Plug iPod into Mac.
4. Open two finder windows on Mac.
5. First window: Navigate to your music media library (possibly "iTunes Media"), specifically to the songs to be copied to iPod.
6. Second window: Open the iPod (the iPod icon, not the other one)
7. Second window: click [Manage Storage] button
8. Second window: click Songs in sidebar. You'll see the complete list of songs currently stored on the iPod.
9. Drag and drop music from window 1 to window 2. There is no indication that anything happened.
10. Second window: click [Done] button.
11. Either window: Eject iPod.
12. Check iPod contents for successful transfer.
Done