How much work would be converting Prusa firmware/software for CNC use?
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I haven't done this myself. But the temperature safeguards only apply to the 4th, the E axis for the filament. So configuring the right steps per mm would get you started.

The question is more what do you want to do with it and where do you get the G-Code to do that. You can not use a slicer to generate the G-Code for you.

But there is software for PCB Milling out there that should work with the configured Firmware.