Don't forget the D800 used. Some can be found that are still like new for substantially less than a new D850. Look used cameras over carefully. There are gems, but some were used as work horses.
I use the D800 as my primary camera, but I find an old D300 works just fine indoors with available light. If you make prints no larger than 8 X 10 you should be able to do "some" cropping.
As has already been said, if your 7200 has a problem with available light, just up the ASA/ISO to what ever proves to work. Unlike faster films, I've never noticed a degradation when going faster with digital, although I've not gone to extremes.

Having said that, the full frame sensors will let you do a lot of cropping, but those cameras are noticeably heavier than those with DX sensors. When on the road, or storm chasing I use the DX format only because it weighs less.
For me, weight is a problem as I've been relegated to one hand shooting and the D800 gets heavier quickly.

One of the advantages of a full frame format camera is that it tends to perform better in low light situations. Given equal size sensors, as with the D7200 and D750, at 24mp, the D750 will perform better in low light because the pixels are larger due to the larger physical size of the sensor. As a result, they gather more light in a given situation. As for which camera has better dynamic range, generally a full frame camera will have deeper dynamic range, but, as the DXO chart shows, that’s not necessarily so universally. My grandson started out with a Nikon D90, and took some amazing photos, but the difference in image quality was stunning when he moved to a D810. Obviously his lenses, such as the Nikon 24-70 f/2.8 and 70-200 VRii contributed to that, but I think you would do well, for what you want to do, with a D810.