If I understood correctly:
-you shoot generally flat subjects
-these can be between postcard size and car size
-they can be high up or very low, making it difficult to shoot straight on
-you want to print to huge sizes
-you want everything to be portable on a 2 hours walk.
I am not sure a medium format is the best choice: the cameras are generally heavy and the bigger sensor means that depth of field will be even more limited to what you are used to. Also: you won’t win that much resolution over your D800.
I am also not convinced that a tech cam is a solution. The shift capabilities won’t allow you to straighten the perspective on the kind of subjects you take, especially with the focal length discussed here. You will need a cmos back to have live view (so as to actually frame macro subjects and focus) and these are limited to 50 mpix (not that much better than your D800). On the positive side, you will be able to take a surface at an angle and move the plane of focus to the same angle (read on Scheimpflug).
Maybe I should describe how I would operate my MF camera on this kind of subject. Not that I am suggesting my camera is the best choice, but to explain what details make a lot of difference.
I use an Hasselblad H4D-50. I has a CCD sensor, so no live view. But the built-in AF system is very accurate, so taking a wall straight on simply means press on the focus button and shoot. All the available lenses for it will give a perfectly sharp image of a car-sized piece of wall at f/8. The resolution is such that I can print 30”x40” and have everything pin-sharp.
On the positive side for your requirements, I can also use a fairly light tripod. The camera uses a central shutter and these produce very little vibration once the mirror is up. I have a very sturdy and heavy tripod and I rarely need it.
If I wanted to take pictures near the ground, I could take the viewfinder off and replace it by a chimney finder. Then, I lose the capacity to meter light (not a big deal on static subjects). The chimney finder is quite light but bulky, so I almost never take it with me on walks.
There is a 120mm f/4 macro lens for that camera. It has about the size and weight of the 70-200 f/2.8 zoom for your Nikon. It gives about the angle of view of a 80mm on your Nikon.
The camera works best at iso 50, and you would use f/5.6-f/8, so you would have the related shutter speed. Only static subjects would do.
There is an adapter called HTS to tilt and shift on Hasselblad cameras. It can be used to shift and keep straight lines straight for architecture (
sample) and shows the same limitations as technical cameras in that case: one needs short focal lengths to be able to shift enough. It is also used for product photography, e.g. watches and food, to tilt and move the plane of focus to the angled plane of the subject. For example, someone explained he used it to image pizzas at an angle and keep them sharp front to rear (you don’t do pizzas, but what you do is of similar sizes). I tried it, it works well, but adjusting focus precisely is difficult (
sample). People shooting watches or food in studio shoot tethered and check on a large computer screen. Outside, looking at a screen in sunlight is difficult. Still: focussing with the HTS is much easier than on a tech cam, because you can actually look at the same image on a ground glass and then just press a button to take the picture. The ground glass is also larger and more comfortable to use than on your Nikon.
So, basically, with the camera I have, I could probably fit most of your requirements. I would use the 120 macro or the HTS and 50mm (second version, possibly with a macro adapter which exists for it). Still: I am not convinced it would be worth the price and effort (for that particular use you have). It would be marginally better than your D800, heavier and bulkier and not easier to use. Other MF cameras would not be better, as far as I know.