[mst]

On the Fuji X-T20, it's very quick to switch from auto ISO to any manual ISO: just push the back dial. It will display the various ISO as well as 3 custom ISO modes where you can predefine values for mininum shutter speed and max ISO value.

Then you likely set this function for the rear dial, as I suggested for the Fn-button above. AFAIK most of the Fn-buttons can be adjusted to set ISO (or many other things). The default behaviour when pushing the rear dial is focus check, though (on the X-T30 too, btw).

Still, fully agree: there is likely no faster way to switch ISO. And the Fujifilm cameras a highly customizable. Not something Nikon can claim for the Z50