I also use a manual speed controller for my router. The J Tech Photonics laser I am working with uses the cooling fan codes to drive the laser at different intensities, similar to the S parameter you mentioned.
I’m just getting it set up and trying to learn how to use it. Once I have a better understanding of what I need the code to look like, maybe I can modify what is generated by image2gcode to work with my setup. For now I’ll get the piclaser demo.