Bluetooth issues

I have an issue with the Bluetooth settings that I can't find a fix for. I have 1 connection that I use to connect to my car for phone calls only and the other connects to my android screen for music, nav etc the reason I do this is because the android screen doesn't use the cars microphone and speakers instead it uses it's own and they are terrible I am able to set the Phone toggle to off for the android screen but it automatically switch's back on causing the android screen to become the default device for phone calls. I have removed the phone permission for the screen but it then makes using the phone via the car's interface very clunky and certainly not handsfree. The screen uses android auto for the connection and I believe this is causing the issue?