Pro tip : if you're on an android phone, specifically a Samsung device, you can download an app called Sound Assistant by Good Lock Labs.
Open it, and scroll down to the bottom setting called Multi sound and set it to the on position.
That should disable the feature that pauses audio from a previous source when a new source appears, eg: music playing in the background gets paused by an autoplaying ad on a website
Also specifically with Samsung, there is a setting called separate app sound. With this feature you can have your music play out a Bluetooth sound source while opening a different application and have the sound come out the phone for that.
It's the biggest feature I miss when switching to pixel.
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.
It depends of what is providing the songs. If you play trough YouTube, it'll detect that the device is off and stop the audio stream, but if you're using Spotify or a different audio player, it could help.
It's worth a try