Trying to simplify and want to pick a Nikon FF body to use exclusively for portraits with a 105 lens. I don't care about age, megapixels, or video. The D2X would be my first choice, but it is a little too old. Of the subsequent full framers does one stand out in particular as doing the best with skin tones and gradations? I use a D810 right now, and it's fine, but maybe a little too clinical.
Is your 105 the Macro? If yes, the 105 1.4 will produce a spectacular improvement for less money than a new body.
Any modern full frame body will do a great job, the lens will make a much larger difference than the body. My favorite all time FX Nikon body for portraits was the D700 (out of four FX camera's that I've owned over the years), but they are a little over priced for the age. Just keep using the D810 you have now, no need to change bodies.
I must say that the d850 images I have seen have not impressed me.. sorry... that weird nikon neon tones are back... in the d850 :-(
I would think that you should stick with your D810 for portraits...
Being a photographer is a lot like being a Christian: Some people look at you funny but do not see the amazing beauty all around them - heartyfisher.
I know post-processing can do wonders, but I'm very old and can already feel Death's icy fingers on the back of my neck, so I try to do as much in camera as possible. "Shoot swiftly and move on" is my working methodology nowadays.
As for the lens indoors, Sigma 85mm f/1.4 Art.
PS: Best portrait skin tones with natural light and strobes straight out of a Nikon digital camera for me was the Nikon D3 which is an absolute steal on the used market today.