My shutdown procedure, key always in pocket:
• Press the P Button,
• Pause music/radio
• Release seatbelt
• Get out of the Car, and lock with the door button (or key fob)
When the driver door opens the car is no longer in READY mode.
If you want to sit in the car with everything off:
Settings (car image, bottom right) > Safety > Power off (still pause music/radio first) the screens will go blank after a few minutes
(unless you get off/on the driver's seat, which will make the car think you're ready to start)