Perhaps this is one of those ideas that some car has implemented and I haven't yet seen it. As many people know, in several years ago a number of cars arranged so that their interior lights would not go off immediately when you closed up the car. This gives you the ability to still see shortly after closing up the car and walking away.

Of course this also drives people nuts, because in many cases you can't tell if the lights stayed on because you didn't close a door properly, and you would end up waiting around to see if they would go off.

Some cars fixed this by having the light fade out, but that's still pretty slow and of course elminates the light you were hoping for.

I would suggest that cars develop some more overt signal, to be triggered immediately when the car has decided that all doors are closed and the car is off, and the lights will be going off in 20 seconds. Such as a quick blink pattern when you close the door, or a flash of the headlights, or a quiet sound or bright internal LED.

Seeing this blink pattern, you would be 100% confident the car is closed and you haven't left the lights on, and could walk away, lit for a few seconds like you want.