The D810 is the only camera I know. Have been using it for 18 months. I am still trying to master my camera. Find that you make better images if you can use the tool correctly in given situations. My photography has improved at the pace of my better understanding of my camera. I have been doing LR and PS for less than one year. In the beginning, I tried, mostly no success, to heroically fix unsatisfactory images with post-processing. Now, my final photos are very much better because I am getting much better images out of my camera.
I looked very hard at the 850. More pixels. Better high ISO. Other things better. But no fill-flash. Low light and night photography should not be better because Nikon did nothing on the low end of ISO. Both at 64. The sensors seem about the same size - but, the programming could be improved/different.
For experts, the 850 could/should make a difference. But I am not there yet. I would like to see side by side images made with each camera, using the same subject matter, settings, technique and post-processing. If the 850 images are meaningfully better, then I will re-think the matter.
Nice shot of your workroom/studio. Very nice space. My Nikon camera + Nikon lenses + peripheral gear and traveling tripod fit into a waterproof backpack and weigh 35 lbs. At this moment, I do not need more or better stuff. I need better knowledge and experience.
Regards,
peter