CarPlay

There is no way to prevent Apple Music from starting when connected to CarPlay. But perhaps a shortcut can be a solution. With an Automation, you can have your iPhone pause the music when you connect to CarPlay.

Try this Automation:

1. Open the Shortcut app

2. Tap "Automation" in the bottom bar

3. Tap "Create Personal Automation"

4. Scroll down a bit and select: "CarPlay"

5. Check "Connects" and tap "Next"

6. Tap "Add Action"

7. Search for Pause and select "Play/Pause"

8. Tap "Play/Pause" again and select Pause in the pop-up menu.

9. Tap Next and disable Ask Before Running and select "Don't Ask"

10. Tap Done.

When you connect to CarPlay now, your iPhone should automatically pause the music. It must be said, I have NOT tested that shortcut, but it's worth a try since there aren't really any other options.