If your iPhone is auto switching to another device, which you don't want it to switch to, there is no reason to leave the other device powered on and within range of your iPhone. That's a pretty simple solution.
Right, because if I don't want my phone auto switching to my car it absolutely makes sense to just... umm... turn off my car. What an easy solution. /s 🤦♂️🙄
When I have a call going on my noise-isolating headset and need to leave the house, the last thing I want is for it to transfer to the lower quality car microphone that picks up all the background noise. The person on the other end of the call always notices. It's disruptive. If I want to listen to music on the car speaker system later I can choose to connect to that at that time. Apple choosing which device to use for me, and not allowing me any way to override that, is asinine and a fundamental failure of UX design. I shouldn't have to unpair my car every time I park just to be able to get in the car next time, start driving, and keep my call going without interruption.