A few additional comments about the Gitzo head. I think I now understand why they specify it up to 200mm.
With the 70-200, it really behaves very nicely. With 500 PF, if I use both hands to hold and steer the lens and camera, it is smooth to pan and tilt and for viewfinder AF it is precise enough. However, if I use live view and pinpoint focusing, the image is a little "drifty" at that magnification (with 500mm). This impression is accentuated if the TC-14E III is also used.
Here is a shot at 700mm, f/8, 1/500s, EFCS, Qc, ISO 2000 (D850, 500 PF, TC-14E III), the image is cropped by approximately 2x (so the field of view is like you'd get with a 1400mm lens without cropping). I used pinpoint focusing on the eye, well at least I attempted to. I think the image quality is acceptable but I would say getting the shot was tricky. I used live view to focus because it allowed me to be more selective about where the focus is placed and the bird wasn't moving much about.
For viewfinder photography with the 500mm PF without TC, I think the head works quite well but perhaps it is at the upper limit of its capabilities, and sometimes one can get some unintended drift in the image if the head is not locked tight (when mounting the QR plate it is difficult to balance the rig so precisely that there is no drift in any tilt position). I do appreciate the light weight of the head and the smooth controls. I think I will continue to use the Arca Z1 + Wimberley Sidekick as my main head for tripod use of the 500 PF, but this is a lightweight alternative when weight is a priority and if one is careful, it can work, but it's not quite as carefree in use as the gimbal setup.