The solution might be to “Disable Absolute Volume”.
See https://www.reddit.com/r/GooglePixel/comments/8hbcuu/the_100_solution_to_bluetooth_volume_issues/ or Audio Issues on Android? Disable Absolute Bluetooth Volume to Fix It - Make Tech Easier
From the reddit link:
Enable Developer Options (options → system → about phone → click build number 7 times)
Press back
Go to settings, and search AVRCP.
Click on Bluetooth AVRCP Version and ensure it is on 1.6.
Next, search for Absolute
Select Disable Absolute Volume and turn this toggle on, if it isn’t already.
Unpair all Bluetooth devices. Restart your device. Pair the devices again. To the right of each Paired Bluetooth device in the Bluetooth settings page is a gear icon, go into each device and turn off the Phone Calls toggle, leaving only Media audio enabled.
Exit settings, connect to the Bluetooth device, start playing something
I’m hoping this will help solve my own problem (on FP4), but I decided to share this information immediately without testing, as even if it would not help me, it might help one of you anyway. Plus, I would probably immediately forget to share this if indeed it will work, and I don’t know how to test it without actually having to setup a call with someone.