I see, so the nut is held captive already in the switch.The button assembly itself holds the nut captive, so one is not required on this part.
The part pictured in the above photos is printed with PLA, which has proved sufficient for all clamps I have printed previously. The only time I ever use PETG is when a higher glass temperature is required.
I used to use a fair amount of PETG whenever I needed something a bit tougher, but modern PLA is much improved in this regard, so I rarely bother anymore.
I would probably still put a little fillet into the area in blue, to avoid stress concentrations at this critical point:
Modern PLA is pretty good, I've made quite a few bike attachment parts from it. It's usually fine so long as you don't hit it hard with a hammer. But this design should be fine in terms of toughness because of the orientation of the layers and there is no space filling.