That can be fixed with Garmin notification sound being treated as a notification sound.

Adding per app volume control is IMHO a really bad UX. It means there's a master volume and then per app volume is like a coefficient the app volume is multiplied by.

My car has its own volume control separate from the phone bluetooth volume and I hate the experience of having two volume knobs interacting with each other when all I want to do is to increase or decrese volume