At some point the phone shuts itself down when it gets too hot. Not ideal to use it as a dashcam, especially if it's also doing navigation and charging.

It's easy enough (for Google devs) to allow the battery to drop to lower charge, and only trickle-charge it. Shut down non-essential apps to keep CPU heat output to a minimum. That might help a little. Not much you can do about the sun though - maybe a liquid-cooling case that holds the phone in place?