Washington state e-bike laws are not adequate for heavy duty cargo bicycles.
The maximum power allowed for a motorized bicycle is 1500 watts at the drive axle. The motor will have to draw more than that.
1500 watts will move 600lbs (total combined weight) up an 8% grade at about 13mph if using the right gear. Unfortunately most people think they need as much speed as they can get. So if a moped bike had a gear box with at least two gears, there would not be a problem.
But if you want to use a fixed gear drive the 13mph is the maximum speed the vehicle would drive at. So the laws should reflect the amount of gear reduction your vehicle has; ie: for 20mph you need about 2200 watts to move that 600lbs up an 8% grade.
The wide torque band of electric motors, particularly the torque available at low RPM, eliminates the need for more than one gear.
Set the single fixed reduction gear at the highest speed that the available power will still be capable of driving your fully loaded vehicle up your steepest hill. Then if you still need a faster gear for less strenuous terrain, you can think about using a two speed gear box.
However it would be easier to just use a gear box, not to exceed 20 or 30mph. And have your vehicle examined for stop-ability with a total combined weight of 800lbs.
The real problem is people's insane lust for speed. They want to be able to turn their bicycles into illegal motorcycles. Well there are better ways to kill your self.