You’re going to need to make some compromises, any of these goals are individually achievable but you’re asking of the off-road capability of a tank, the portability of a penny board, with the top speed, price, and range of a mid tier AT board.
If you want off-road capability you need:
1: pneumatic tires which means very little range on a 99wh pack and wouldn’t fit in a backpack.
2:SR125 wheels which may fit in a backpack, but would also make for less efficiency and wouldn’t get the range you want.
If you want to climb a 40% grade, you’ll need incredibly wide tires, 4WD, very large motors, a very powerful battery and powerful VESCs. Which has 0 chance of fitting in a backpack, will quickly surpass your budget and also couldn’t be powered off of a 99wh battery.
I’ll use examples as well, if you’re looking to build something portable with impressive range, @Pecos built Blue Koi
@sevenfloorsdown built Boardup V4
The only things I could see climbing a 40% grade are things like @Dinnye’s RaceBoi
Or @poastoast’s Stop Resisting
None of these builds have flight safe batteries though, that’s really tough to build around IMO