How do you stop Apple Music from automatically playing in your car?

I suggest a different approach. How about saying “hey Siri..music pause (or stop, or off)”. On my HomePod I can say any one of those commands and it’ll stop the music. I don’t know if all of them will work for iOS(I don’t use Siri for music on iPhone), however, “hey Siri .. pause music” should work. And it accomplishes what you want: leaves music playing when you want it playing, stopped music other times when you don’t want music, it leaves the phone volume up and Bluetooth remains connected, best of all you don’t need to be concerned about the auto play.

Turn off Bluetooth

That's right you'd have to jack into a USB port for audio from your phone. That's what I have to do. It sucks I know and every update apple makes things worse and worse