Considering Android Auto is basically projecting your phone onto a car display it does make sense for the icons to match.

Icon packs are a "power user" thing but I wonder how hard it would be to put custom icons in via root or revanced type project

Honestly this kind of thing is a junior engineer's week or two. Switching up icon sets is simple, and has value for users who will more easily understand what's going on if it's using icons they already recognize.