Your shooter is definitely way too fast. With a 1:1 ratio off a 20,000 rpm 550 using a 4.875" diameter wheel, you end up with a
425 ft/s tangential velocity. That's 290mph, and there's no way your shooter is accelerating the frisbees to even close that speed. It would be much better to slow it down and increase the torque. Because of the extra torque, the speed will drop less as you shoot, and your shooter will more effectively accelerate the frisbees.