Pretty simple suggestion, but I think a reduction to the time it takes for the crank gun to begin firing when winding it up would make a huge difference. Vermintide is a chaotic game that punishes staying in one place for any period of time and that should be reflected with his ult. I would even suggest reducing the movement speed penalty of the wind-up but either one would make him feel much more viable on high difficulty.
Isnt his level 30 middle option exactly that?
The middle one reduces the time for the crank gun to reach full firing rate but not the time for it to begin firing.
Fire delay is only 0.5 seconds. You should be using your dodges to stay mobile while firing.