https://unix.stackexchange.com/questions/437468/bluetooth-headset-volume-too-low-only-in-arch
VMG's answer is subtly wrong; it will technically work, but it will disable all other plugins than a2dp, meaning bluetooth keyboards/mice/gamepads/etc will stop working, when the only plugin causing issues seems to be one called avrcp.
Edit
/lib/systemd/system/bluetooth.service
and change
ExecStart=/usr/lib/bluetooth/bluetoothd
to
ExecStart=/usr/lib/bluetooth/bluetoothd --noplugin=avrcp
and run
sudo systemctl daemon-reload
sudo systemctl restart bluetooth