That would work.
You could also use a couple of your opto outputs to step a stepper motor, but the RC servo is a better solution because it contains position feedback -- so the pot will always be driven to the same position for given analog voltage input from the controller.
What is the point of this "pot turner"? You seem to imply that you're trying to control the cut current of a plasma cutter from the CNC, and per the folks who make the plasma cutters, including the hi-def ones, controlling the current during CNC cutting provides little to no benefit.
The plasma cutters don't have such a control input (at least the typical air plasma units don't), because there is no need for them.
Many units do have a control input for "arc on" and an output for "arc good" and an arc voltage tap as standard equipment, because those are the control signals required for CNC control, but arc current is not needed as a dynamic control.
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