Holding torque.
Your holding torque is the stepper torque multiplied by your pulley ration.
Make sure the driver you are using does not have the current reduction feature enable, other wise the holding torque will be reduced considerably.
Set the current limit to the highest setting, consistent with not exceeding the specs of the motor, driver or power supply.
You won't do any better than that without more reduction, or a holding brake.
Professional units use a drag brake that is strong enough to take all the cutting loads, and an encoder and a servo to move it with brute force.
That gets rid of the backlash. There is none.
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.