Don't waste your money.

The IQ of the D810 was astonishing when it came out, and it can still go head to head with any newer full frame camera. The difference in IQ and detail between 36 megapixels (D810) and 45 megapixels (D850) is not as great as the numbers would suggest. I say that from personal experience. For a brief time, I had both cameras in my house (before I gave the D810 to my son in-law). With my best lenses, taking the same photos at the same settings, I could see just a bit more detail in the pictures taken with the D850, but only pixel peeping. In the real world of looking at pictures on a computer screen or making reasonably large enlargements, there is no difference. So for portraits and fashion, forget it.

As for upgrading to the D850 for sports, it's a maybe. It's a faster camera, faster autofocus, better focus tracking etc. It has a marginally faster frame rate than the D810 but significantly faster (9 fps) if you use the battery grip. Think of a beast of a camera with a beast of a full frame lens on a monopod on the sidelines and you have what you need for sports with this camera. I think you'd be better off using the Z6ii, taking advantage of the additional "reach" you get with full frame lenses.

The only sensible upgrade to the D810 would be a full frame mirrorless camera. The only compelling reason for not going that route would be if you own many older autofocus lenses that require a motor in the body for focusing and don't want to replace them over the cost issue. The FTZ adaptor would not provide that.

I have a D850, a Z6, a Z7, and a Z9. The D850 is easily better than the Z6 & Z7. Except in extreme low light and resolution the Z6 & Z7 are essentially the same camera. A very good prosumer camera. I get extremely good results with both, especially the Z7.

The D850 is a better camera than either of them in terms of its output image. The Z9 output isn’t that much better than the D850’s - it just is much faster at getting it done. It’s also easier to configure as you want it - more buttons and a UI designed for some who knows what their doing IMHO. It’s also more unforgiving too.

I’d get the D850 it’s a really good camera.

Actually, don't underestimate the ability to crop with 36 megapixels. Download the landscape and then zoom in and explore the picture. Can you find the blue tractor below the trees toward the left upper picture? Can you see the detail in the sheep? There is plenty of detail for cropping. Also look at the picture of the people walking and the modest crop. You can easily read the printing on the bags. These pictures were taken at Birdoswald's Roman Fort at Hadrian's wall in Northern England. They were taken with a 50mm f/1.4 AF-D prime on a D810. I've posted these pictures before, but I want to do it again to make a point. I don't think that the advantage with 45 megapixels is as great as you think. I've owned and used both cameras. The OP should be looking beyond the obsolete D850 to mirrorless and get the 45 megapixels anyway. I don't have a problem with more pixels, but expectations should be realistic.