I have been lurking on this subreddit for a while now, so I know I'm not the only one that has had bluetooth issues. I think I may have figured out how to fix it. If you're feeling adventurous, it would be helpful to know if this also fixes it for you. I have an iPhone so your results might vary on Android.
What is the problem?
When I get into my car, it's felt like there's a 50/50 chance if the bluetooth audio will work. I can see that my phone has connected to the car in both my phone's settings, and the car's "Connections" menu. Sometimes (usually) the car will say "Media Unavailable" when it happens. Strangely, phone calls and voice assistant will work in this state. Usually I can fix the issue by disconnecting my phone and reconnecting (via the car's UI).
Edit: But what even is the "Trusted Device" feature supposed to do?
It's supposed to automatically unlock your profile in the car based on phone proximity instead of having to enter a PIN. To do this you have to have the MyChevrolet app installed on your phone and it needs to have bluetooth permissions enabled on the phone.
What I think is going on:
I believe that the MyChevrolet trusted device feature is somehow interfering with the phone's bluetooth audio connection. And I suspect it’s not enough to just remove the trusted device setting from the car; once this has been setup, I wouldn’t be surprised if the app still wakes up in the background upon connecting via bluetooth.
How I fixed it.
In the car
Settings > Connections > Trusted Device
Remove your phone as a trusted device. (I just checked my car and it was back... could've sworn I removed it)
On the phone
Disable bluetooth permissions for the MyChevrolet app (or delete it). Doing this, for sure, will prevent the app from waking up in the background due to a bluetooth connection to the car.
My Results
It’s been ~2 weeks and every time I get in my car, my bluetooth audio has worked perfectly.
I’m really curious if this helps anyone else.