You could go hunting for an appropriate digital potentiometer IC, aka digipot.
Or you could go with another low tech solution, such as relays attached to fixed resistors to pick different power levels. You could use one relay for each power level you need, or select resistor values such that you can pick a range of power levels depending on which combination of relays are active.
Or the optoisolator/transistor equivalent.
The solution you already have will likely work too. You've created a motorized potentiometer, which can be found as a commercial product as well.
Or you can figure out how the control circuitry on the main board actually works and skip the child boards.
At one end it says "clk & Data"
The labeling on that end is clearly for a 5-pin connector, in which only a 4-pin connector is installed. So who knows if they apply logically or not. One would have to know more about the circuit as implemented to figure it out.