Phone keeps setting the default Bluetooth device for calls to the wrong device

I have an issue with the Bluetooth settings that I can't find a fix for. I have one connection that I use to connect to my car for phone calls only and the other connects to my Android screen for music, navigation, etc. The reason I do this is because the Android screen doesn't use the car's microphone and speakers - instead it uses its own, and they are terrible. I am able to set the Phone toggle to off for the Android screen but it automatically switches 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 hands-free. The screen uses Android Auto for the connection, and I believe this is causing the issue.