PWM equivalent for a constant current source and load
Mains-powered constant current drivers will only work as current source inside a certain voltage range (see the label) and they will have a capacitor on the output, so you can't use a shorting FET to apply PWM dimming. They're more like constant voltage supplies whose voltage is adjusted (not very fast) to keep the current constant, which only works if the load isn't changing too fast...

You need a dimmable driver, with a 0-10V dimming input, which you can use to control it. Or something DMX compatible if you have lots of lights and a DMX controller.