We have indeed found out by now that the Bluetooth issues are connected to the Pi’s management of its built-in BT module. If there is too much of a load on the system, dropouts are more likely to occur. The same goes for WiFi. the Pi prioritizes the BT connection behind other processes which can lead to issues in BT playback. The easiest solution would be to get a BT dongle. We have run test with both dongle and built-in and only the latter ever drops out.
Unfortunately, hardware volume control is only possible with (most) HAT sound cards on our OS.