Hi again! After a long time I thought I should finally answer my own question, since I finally found a lens (for a reasonable price) that I am happy with:
Nikon 150mm f/5.6 Nikkor-W Lens does the trick for me. It is very sharp, and never blurry towards the edges. I have captured some very nice images with the Optar lens too, but with the Nikon I don't have to worry about anything turning blurry towards the edges.
One minor bad thing is that the Nikon does not work with the inbuilt shutter mechanism of the Crown Graphic, so I always have to use the cable release. But that's OK with me for now.