CarPlay audio volume and Siri volume

I have moved to iPhone 14 Pro mainly to switch to Apple CarPlay. I was previously using AndroidAuto for many years. One thing i'm finding to be bizzare and silly is how Apple have made controlling CarPlay volume vs Siri Volume.

The problem is this:

I'm listening to music on Spotify via CarPlay. Volume is set to comfortable level. I then have to invoke Siri to either read a message or compose a new one etc. Now, Siri volume is awfully low and I can hardly hear him. So, reading on the forums I have read that apparently on factory CarPlay headunits that are officially supported by Apple if you adjust the head-unit volume while Siri is talking, then this should invoke the iPhone to only increase Siri volume.

I tried doing this, but it does not work. As soon as Siri finishes talking my music resumes and the volume of my music becomes very loud.

On AndroidAuto, while something is playing via AndroidAuto you can use the physical hardware volume buttons on the phone to adjust the volume. So if I was listening to music on Spotify via AndroidAuto, simply adjusting the volume using the phone volume buttons will adjust the AndroidAuto audio output volume. Then when I invoke google assistant, i can do the same and adjust google assistant volume. This way I can get a good balance of the two volumes.

Now the question is, why on earth have Apple restricted the use of the physical hardware volume buttons to adjust the volume of whatever is currently actively playing? at the end of the day, regardless of whether the head-unit is supposed to be able to adjust it, the true controls should be on the phone too because it's ultimately a phone preference and Apple are not allowing us control over something so basic.

This is on a Teyes CC3 head unit. I have tried it on 5 different brand head-units on different friends cars and all do the same. I have to yet find someone who has a factory head-unit and see if it works on theirs. Then maybe I can hook it up on that and make the adjustment and hopefully the phone remembers it.