To maintain high torque at low RPM, use an rpm sensor and PID controller software. The PID controller adjusts the PWM to achieve the required rpm. If necessary it will apply full power (100% PWM) to the motor until it is up to the required speed.
I have been able to control the speed and direction of the motor by PWM. But as the RPM of the motor decreases, the stall torque of the motor decreases as well.