If I were to spend 30000£ on a fisheye lens, I would save for the big fish (6mm/2.8 ), much better optically than the 6mm/5.6.
Otherwise, for 1/60th of that amount and with the same amount of inconveniencies, I would save 25000£ and put 500£ on a 7.5mm/5.6
BTW, that's what I did about two years ago, and I even chipped it... It goes nicely on any FF with the F bayonet. Just do a MUP, mount the lens and turn off the camera. The mirror will sit pretty on the side of the protrusion and not get damaged at all. Not to be attempted with the 2.1 cm/4 !!!
Can also be done with the 10mm OP, as Birna has shown countless times (can be done on the 8mm/8, too, but the 7.5mm/5.6 is better).
If the camera has LiveView, it works about the same as a Z6/Z7.
To remove (you have to have sufficient battery power, above 80%), do another MUP and unmount the lens.
Attaching a photo of the chipped 7.5 mm
Ciao from Massimo