Computer: RasPi 4 8GB + 7” screen with Ubuntu Mate in a SmartiPi case.
Car: put the computer onto the visor and flip the display, hook up accessories.
I’m still with iPhone but pretty close to that setup. Mainly I am thinking about emergencies where I want my SIM in my phone when I leave my car. A second SIM that supports WiFi calling is likely the final step, giving the new # to the iPhone and using WiFi only when possible, at home or in car.
Sounds complicated, no doubt. Tethering CarPuter to iPhone is a much easier approach. I’m a VanLifer.