Thank you for the suggestion. The switch body is plastic. I'd be tightening the M10 nut on plastic threads. It wouldn't take much torque to strip the threads.
Also, the shaft appears to be plastic too. Even though it's available with a flatted shaft, I'd be tightening the knob's set screw into plastic. Not good engineering practice, and the plastic shaft would swell and stick in the knob.
So do you think there is metal option for a similar thing?
The plastic (nylon) is tough. You won't strip any threads by securely tightening its fixing nut and the set screw would certainly bite into the shaft but it won't deform it. I've never had issues with plastic parts similar to these.
0.2 amps is a very small current, about the same as a 50 watt mains bulb would draw on 230 volts. And that is an absolute maximum 'resistive' rating.
An on/off switch for an amplifier needs to be rated high enough to withstand not just the steady state current but also the arcing that occurs each time the contacts make and break as they switch the highly inductive load of the transformer.