Question Remotely controlling music... from a Smartwatch... on Android head unit?

[ethal]

Hi, all! So the title pretty much sums it up. Basically, I was hoping on using my watch to control media in my car, where I installed an android head unit, with hopes that the watch would double as steering wheel controls...

I have music files and Spotify on the HU, so naturally I tried using Spotify to control it, along with the local files, but since when does Spotify tablet edition not have a volume slider? The watch is able to control the phone, of course, as well as adjust the volume on windows laptop, but for the HU all I get is the basic next/previous track and stop/play.... is this a dead-end? Android auto same thing - the watch controls the "media" volume on https://vlc.onl/ the phone, but not the "Spotify" slider in the volume mixer.

Currently the best I can do is use the HU as "Bluetooth audio" device, and the phone is forced to control everything watch and audio related, but I really hoped I could use the HU as the central hub...

Am I wasting time here? It being Android I'd think there must be a way, since the actual communication infrastructure is all there... Perhaps there are other app recommendations I could try?