Have you checked out the GFX 50 Facebook group? It has many,many owners wrs of the GFX 50,and they have had a lot of discusssions about the camera, the lenses available for it, and the many lenses that they have adapted to this fine camera. I was in the groupo for a couple of months, and saw many fine photos made by the GFX 50,with both the primes Fuji has released, the zoom, and MANY different prime lenses from all the major (and minoir) lens makers.
A HUGE number of lenses originally released for 35mm cameras can be adapted to the GFX 50, most with very minimal or no fall-off in the corners!