I did some further investigations yesterday initially with just the built-in Bluez tools “hciconfig” and “bluetoothctl” together with a Logilink BT0015 Bluetooth 4.0 USB dongle. Looks like Debian’s connection manager “connmanctl” does not properly support this, i.e. there was an error when I finally tried to connect to my mobile after successful pairing, even though connection was possible in the end. Also I want to avoid the connection being NATted as the cm might do at least, but instead let the bnep0 interface get a DHCP address directly from the Fritzbox. Obviously this needs some more effort, but from current perspective the mentioned pi-btnap looks good there.