I had problems like this trying to get Spotify to be the app that opens with Carplay/bluetooth. Whenever I install Apple Music, it becomes the default. Apple doesn't even have a way to change defaults for music player or siri.
Found this about Siri default player: https://www.imore.com/how-set-preferred-music-service-siri-iphone-and-ipad
I had problems like this trying to get Spotify to be the app that opens with Carplay/bluetooth. Whenever I install Apple Music, it becomes the default. Apple doesn't even have a way to change defaults for music player or siri.
Found this about Siri default player: https://www.imore.com/how-set-preferred-music-service-siri-iphone-and-ipad