this seems like a potential area for innovation; Self propelled consumer ICE lawnmowers are plagued by open differentials, front wheel drive, and a complex drive system (belt and clutch attaching to a constant speed motor)

Electrics, on the other hand, could easily have independent front and rear motors, if not each wheel; an independent ramping and traction control system from the main motor -- and even an isolated electrical system (via capacitor) such that grass load and wheel speed are indepedent.

Is that a large market segment?

Everyone who can't justify an electric riding mower or doesn't have a place to charge it. I'm guessing you've never had to maneuver an umpowered push mower on slopes.

No, I have, but I was young and am not small.

But for instance, the town here doesn't have slopes, which is a common thing, for developed land to have been flattened out.