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
I really prefer it for having different things set at different volumes. Like presets.
I'm surprised Pixels don't; small things like this area each QoL wins.
Samsung/OneUI has a good few addons like this. Ability for multiple apps to play sound is good too (same addon) and the ability to output different apps from different sound devices at the same time (built in)