This is a great idea but just something to keep in mind when comparing spur gear direct drive systems vs belt systems.
In a belt driven system, you have multiple teeth, usually a minimum of 6 meshing with the belton the pinion gear and about half the pulley on the wheel engaged as well. This will distribute the forces across more teeth which in turn means less wear per tooth.
The belt and the gear both move in the same direction and there is no slippage(unless you’re over torquing and the belt skips). This also means less wear and tear.
Should you fuck up royally, you strip the belt. A $5-$10piece.
With direct drive gears you’re engaging 1 tooth at a time. You will only ever have 1 tooth pushing against 1 tooth. Best case scenario, you got the spacing perfect and you have 2 teeth during the transition between teeth but that’s going to be a lot more strain on the teeth which will cause more wear. Although the tooth profiles are larger so that should mitigate any real issues there but you may require regular greasing and cleaning.
The teeth are supposed to roll over one another though you’re broadly going to have some fictional losses there from that not being the case. Are these losses more significant than those on a belt system? Probably not, it might actually be a little better. But it will more likely be significantly louder, which may not be an issue on a all terrain board you want to use out in the woods but rolling down a suburban area or a peaceful park may annoy some pedestrians.
My last concern though is a more practical one which is if for whatever reason you brake really hard or you somehow managed to lock up your wheel at speed then the gears have nowhere to go. All the stresses get transfered to your mount which may warp or break it depending on how sudden it happens. Don’t know if that would be an actual issue to be concerned about since I think you’d have to lock up the motors for something like that to happen which probably wouldn’t happen.
Those are just some thoughts though, I like it. I think you’ll get some serious power transfer out of it