oh, sorry, i thought you were not recommending that build based on the comment "You have 10 minutes of this torture until it melts."
No, i'm recommending it, it will handle your most extreme grade for up to 10 minutes. I know your area, 20% grades don't last long.
Adding ferrofluid to the motor would be required to do this.
you also comment "it will take off like a rocket." what exactly does that mean? my kids might love the rocket like ride, but it seems a bit unsafe to me. Though i assume the throttle/peddle assist can be controlled to make the standing start more reasonable?
Yes, the same power level that allows you to take off like a rocket is *just barely enough* power to let you climb a 20% grade for, let's give it a safety buffer and say 5 minutes. With controller tuning, you could reduce the torque from a stall so that the bike isn't doing power wheelies, but still has tons of torque in the mid range ( the hill climbing power ).
Some controllers also have a up/down button on the screen for increasing and reducing the power level. This way, you can get out of programming the controller to reduce the overall torque, then increase the power level when you hit a hill.
One of the main reasons for choosing a hub motor is for regenerative braking. Going down a hill with a cargo bike with a ~300lbs load is a huge problem. A regenerative brake does not fade and lose power due to heat, it converts motion back into battery energy, which you need because these hills will consume tons of power. It also can be tuned to be stronger than the strongest disc brake, with a big enough motor.
You have a huge problem with the design of this bike. The rear doesn't look disc brake compatible. You don't want all your powerful braking on the front when descending a 20" hill. The bike will want to send you flying. What you actually need is the strongest rear brake you can get, a 203mm disc, but if the bike doesn't have a native connection for that, a disc adapter would be too weak to handle the force of such a large brake.
A quite high power hub would be the only safe way to get up a hill like this, and not die on the way down.