since rolling resistance is so low, a smaller rear tire?

as long as the driving wheel is in front when cruising below 5% on inclines and both rear and front wheels drive on inclines above that, i don't see much of a downside to this. except 2 different rpm motors maybe and 2 throttles or a complex controller.

does the weight shift to the rear wheel motor the steeper the incline you ride up?

and for when running into steeper inclines, wont carrying a mid drive only be beneficial in the long run? because maintaining speed for more mass is easy, and every other riding condition / scenario is heavy resistance or acceleration which a mid drive is far more efficient for?

For a hub motor, smaller tire,, yes please. The higher rolling resistance of the small wheel is nothing compared to the greater torque it has. This is for paved riding.. Off road, you might need to roll over a rock staircase, and a 20" wheel will just make you bust out your teeth on the rocks.

For a mid drive, you can simply shift down, and get the motor rpm back to its more efficient rpm. (70% efficiency is fine)

The steeper the hill, the more your weight shifts to the rear, up to the point where you shift the weight past the rear wheel, and wheelie off. for me, this is about 20 degerees. degrees, not %. I shift thinking to degrees when it gets steep, because everything in ski mountaineering was in degrees. 20 degrees is blue square rating ski slope.