I believe you’re referring to Terminal Velocity

You seem knowledgeable with these workarounds, would you happen to know of a way to bypass ducking?

It’s the temporarily lowered volume for notifications etc. I know app developers can choose how to implement this, PowerAmp lets you choose whether to do it or not. I can’t find such a setting for Audible and Spotify, and while navigation prompts are played through my phone (on purpose) it still lowers the music on the car’s speakers.