I use the Op/Tech Dual Harness, which is modestly priced and easily enables me to carry two camera, one of them a large Medium Format analogue camera, for hours without discomfort.
I review the harness as used with the Pentacon Six here: http://pentaconsix.com/p6straps.htm
I have now added the Op/Tech straps to my GFX 50S, threading the straps through the clips that are supplied with the camera, and this works very well.
The Op/Tech Super Pro B connectors look very similar to the strap
I have three of the Hartblei Pentacon Six to Fuji GFX adapters: simple adapter, shift adapter and tilt adapter.
With them a series of longer lenses can be used. For instance the old Enna 400mm lens is excellent on the Pentacon Six and so will easily cover the GFX format. See a review of it here: http://pentaconsix.com/enna.htm
There are two 500mm lenses for the Pentacon Six, the Pentacon lens, which is very heavy (3.5 kg) but excellent. See here: http://pentaconsix.com/500_560mm.htm
Review of Laowa 17mm & Fuji 23mm GF lenses
First, the Fuji 23mm f/4 GF lens for the GFX cameras is excellent in all respects. There is very slight vignetting at maximum aperture, but that is to be expected with such a wide-angle lens.
The 17mm Laowa lens is not in the same league. No EXIF data on aperture transmitted to the camera, and there is significant vignetting at f/4 and f/5.6, with some vignetting even at f/11 – visible in test pictures of a brick wall, but not obvi
I have now tested three further lenses with the Hartblei shift adapter, using the same settings as with the above lenses. 55mm Arsat shift lens
Zero shift: Resolution good over most of frame. Some loss of resolution near the edges. Some chromatic aberrations.
Lens only fully shifted up and fully shifted down. Resolution OK.
Lens only fully shifted left. Very good resolution (although any defects may have been masked by out-of-focus tree branches nearer to the camera.
The discussion on this topic is as old as the X-Trans sensor itself. It is now a well-established fact that Capture One and Silkypix Developer Studio offer the best approach to read out the correct sensor data, unless you only use Fujifilm X RAW Studio. You can certainly get good results in Adobe LR too, but the handling of the X-Trans sensor is different than in the three converters mentioned.