Shaded pole speed control with PWM
It is not surprising that a variable voltage speed controller does not work very well with a shaded pole motor. At full voltage, peak torque is likely only a little higher than the full-speed running torque. The motor is not very efficient at full voltage, so the losses when the motor is stalled at reduced voltage may not be much different from the losses under normal operation. However, there is no air moving over the motor when it is stalled or running at a very low speed. The motor temperature may rise above the safe limit, that will certainly shorten the motor's life, but probably not cause it to fail very quickly.

A different voltage control method will not help at all. A variable frequency control should work reasonably well, but there are likely none available for a motor like that at a reasonable price.