The movements on a crown Graphic are pretty sparse, as you have no back movements Nor twist on the front of the early ones. They were after all press cameras, not technical cameras. The Optar is not a bad lens Just not up with the best German ones of the day.
As a press camera lens they would have been used mostly at the F5,6 f8 Only when movements were needed would they have been closed more.
The apparent vignetting you see, is far worse than you will get on the film. mostly it is a characteristic of the ground glass. and it seems much the same on all lenses. but gets worse with wide angles.
A Xenar or Tessar lens would be a good choice as they were far more widely used.
An Angulon wide angle must be used very well stopped down, Unlike the later super Angulon, however even that can only be used for focusing wide open.
The wray wide angle used for the early MPP is as good as the angulon but like it, it needs stopping down.
Most shorter focus lenses do not have an image circle much bigger than the 5x4 film. so when you raise the lens you are actually moving out of the area of coverage, rather than normal vignetting. Stopping down will not help in this case as the image extends no further.