There is at least one camera with a 5:4 aspect offered in-camera, the OP's Nikon D850. Apparently, many don't have that option, though, so your markings make good sense. I find it hard to visualize a crop ahead of time also, so I often have my native 4:3 Olympus set at 3:2 for landscapes.
Kozan wrote:
The only problem i see is that if you want to print 8 x 10 but your in-camera settings are for 3 x 2 format, you may not be able to crop to 8 x 10. Some camera clubs require 8 x 10 format. It is frustrating trying to print a 8 x 10 picture and being able to crop properly is just not there. I have since marked my 3x2 screen with the 8 x 10 lines so I know where to take the image so I can crop.