Some simulator stuff to help you out. Some things are guesses, like your battery. You say it's 20s11p, but you do not specify what cells or BMS are used, so no way to do more than guess what it is capable of or it's capacity. So for this purpose I'm assuming 2.5Ah cells, which 11 * 2.5Ah = 27.5Ah; and it probably is capable of at least 2C continous, which would be about 60A.
This uses the 4506 (the motor you specified) with a 72v battery, and a 60A controller, going up a 30% slope with 265lb total (165lb rider and 100lb bike).
https://www.ebikes.ca/tools/simulator.html?batt=cust_72_0.2_27&cont=cust_60_120_0.03_V&axis=mph&hp=0&autothrot=false&throt=100&mass=120&grade=30&motor=MX4506&wheel=26i&frame=mountain
That gets you only 11.3mph full throttle up that 30% slope, with Motor Power 1792W, Battery Power 3595W, Batt Amps 59.9A, Batt Volts 60.0V due to voltage sag.
Power Consumption is 318.1 Wh/mi, and with that pack size you only get Range 5.1 miles if you're on those hills exclusively. (which you won't be, but if there's a lot of those hills, it's going to eat your battery).
But before you ever get that far, it'll melt the motor. It'll Overheat In 2.7 minutes of riding on that steep a slope. If you had statorade in it, you get three whole minute total before it overheats.
Overheating defined as possible permanent damage, greater than 250 degrees C final temperature. This is becuase the difference between battery watts and motor watts (actually "load watts") is all turned into heat in the battery, controller, but especially the motor.
The below is for 20% slope, otherwise all the same.
https://www.ebikes.ca/tools/simulator.html?batt=cust_72_0.2_27&cont=cust_60_120_0.03_V&axis=mph&hp=0&autothrot=false&throt=100&mass=120&grade=20&motor=MX4506&wheel=26i&frame=mountain&cont_b=cust_60_120_0.03_V&motor_b=MX4506_Ferro&batt_b=cust_72_0.2_27&mass_b=120&hp_b=0&grade_b=20&add=false&blue=Lbs
That gets you up to 17.7mph, Mtr Power 2054W, Batt Power 3081W, Batt Amps 49.6A, Batt Volts 62.1V (still sags a lot in voltage), power Consumption down to "only" 174.1 Wh/mi, and Range up to 9.6 mi, again assuming only uphills, but the Overheat In 5.0 minutes means you still wont' be able to use even the single charge of the battery before the motor melts.
If you're just riding on completely flat roads (which it doesn't look like you even have :lol
then you'd get this:
https://www.ebikes.ca/tools/simulator.html?batt=cust_72_0.2_27&cont=cust_60_120_0.03_V&axis=mph&hp=0&autothrot=false&throt=100&mass=120&grade=0&motor=MX4506&wheel=26i&frame=mountain&cont_b=cust_60_120_0.03_V&motor_b=MX4506_Ferro&batt_b=cust_72_0.2_27&mass_b=120&hp_b=0&grade_b=20&add=false&blue=Lbs
Which gets you up to 29.2mph, using a measly Mtr Power 938W, Batt Power 1072W, Batt Amps 15.6A, Batt Volts 68.9V (not nearly as much sag), and a decentish power Consumption of only 36.7 Wh/mi, which with that size pack gets you up to Range 50 mi and never overheats, at a lowly 66 °C motor temperature.
Your actual conditions, controller, battery, weight, and riding style will change all those results.
Personally, I'd go with a middrive you can shift gears on, which would let you shift down to trade speed for torque to climb the hills, and then shift up to go faster on the flats. It'd still have to be a fairly high power middrive to go up a 30% hill at much more than a crawl, and would probably wear out the sprockets and chains kind of often under those kinds of loads, but it'd take a lot less power and shouldn't melt down.
