Another update to the source code.
https://create.arduino.cc/editor/tfabris/4c5eea9a-1462-45d7-908d-81a5bb6b0d90/preview

This makes it so that the same set of notification commands is always sent to the bluetooth chip every time a track changes or the playback status changes. This was an attempt to fix the bug with the Kenwood headunit where it only ever got the first track title on its screen and never got subsequent ones.

It didn't fix that bug, but it fixed another previously-unmentioned bug where I had a bluetooth headset which wasn't successfully pausing/unpausing when I pressed its pause button. Something about the change in the last set of code fixed that bug, and now the headset correctly pauses and unpauses.