I cannot speak to the D850 specifically but I believe that generally speaking, lenses get better with increasing resolution sensors if you look at the final images of the same size. My experience is with D90, D7000, D700, D600, D750, and Z6 on which I have used the lenses below.
I would strongly second the recommendation for a 105/2.5 lens if your ultimate goal is to get the 105/1.4. I have the Nikkor P.C. with scalloped focus ring. This lens is simply amazing. There is a thread somewhere by Olivier Anthony who took an extensive series of excellent portraits with this lens, I believe first on the D700 and later on the Fuji XT-something, but I cannot find it now. Maybe you can contact him
https://nikongear.net/revival/index.php?action=profile;u=34. Since the lens can be found so cheaply, why not simply try it out? Otherwise I can upload some test shots later, I'll see what suitable subjects I can find (human or not). Most of my shots with this lens are not portraits, I use it for everything including architecture
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If you need AF, why not an AF 85/1.4 D or an AF-S 85/1.8, depending on the budget? I think the former can be found between 500-700 Euros and the latter around 200-400 Euros used. I do have experience with the 85/1.4 D. I don't use it much nowadays but every time I take it out of the drawer I'm astounded by it's beautiful rendering. It has some issues with LoCA that you need to work around depending on the conditions and if you are using apertures larger than f/2. I often use the lens from f/1.7-f/2.8 where it shows it's best IMHO. The reason I don't use it much now is because the 85mm AOV is not the most natural for me, I prefer longer (105, 135, and 180) or shorter (35, 50) most of the time. I found the lens more natural on DX than on FX, but that's just my taste.
The 180/2.8 ED Ai-s is very nice but the focal length is more niche, I would say. I use this more for concerts etc. where I need to get a tight shot from a fair distance. In any case, it has an old-school rendering which is soft and pleasing, of course the lens has quite high resolution as well, but the images look less harsh than modern designs.
Finally, the 105/2.8 AF micro (D or non-D) can be had for about the same price as a 85/1.8 AF-S and is also not bad for portraits if you can live with the slower aperture.