New iPhone not recognized by Apple Music on macOS
It seems that maybe Force Quitting AMPDevice*Agent may have resolved this issue. Attempted to restart, and got some error about Finder blocking restart because of activity with iOS device. I saw my old iPhone (no longer in my possession) in the Finder, and wasn't sure what to do. I Force Quit that one task, and restart succeeded. On restart, Apple Music no longer listed my old phone and now listed my new one.