thanks @Andy Work for the tip, the Lens Carrier System is indeed interesting. Similar concept to the Cotton Carrier Harness I currently use.
Thanks @arbitrage for your input. It is crazy how similar you and I go at analyzing the situation and assessing the options. I too believe 200-600 and 800mm are not going to cut it (weight wise). I will also try to avoid using a TC and instead work more on my technique getting closer to my critter without scaring it off. I am however willing to sacrifice on MDF, since I now always carry the Z50 with me, to use for close ups and Landscapes. So MFD is not a deal breaker like weight.
In fact, ff I use the 500 PF with my D850, it gives me flexibility of focal distance through the Crop-factor. I can also use it with all 3 cameras. I think I will wait on the 400 PF *at least the weight and F aspect to be published) and will then chose among the 3: 100-400 S, 400 PF, 500 PF.
Just out of curiosity, anyone out there NOT happy with the 500 PF?
.
.
The only person that I know of that wasn't happy with it was Mark Smith (via his videos), he said he had a hard time getting good images but I'm going to say he got a bad copy because I really haven't seen anyone else complain (I thought it was great while I had it).