Low light specifications are like a lot of other specs. They really are useful if understood and interpreted correctly, but end up being a trap and source of disappointment for those who misuse them. The simple fact is that even night vision goggles cannot see in the dark. All sensors and all photographic films need so.e quantity of photons in order to perform their intended function.

Too many people see or hear about a half stop improvement over a previous model or different camera and just literally go nuts over it. In fact, that difference may be detectable in real life or (more likely) it may not.

There is nothing wrong with trying to get a camera to do something beyond its inherent capabilities. I frequently play that game. I used to do the same thing with computers, and before that with stereo equipment, and before that with handheld calculators. It's a lot of fun.

Living "on the edge" is a popular topic today. But living on the edge involves uncertainty.. no guarantees. Sometimes the edge of the cliff gives way, sometimes it doesn't. And you almost never get to choose when that happens and when it doesn't. ( Have you watched any of the Olympics the past couple of weeks?)

I suggest that those working at the limits of their cameras' capabilities just take a breath. Realize that you are in a risky area. Expect that the camera with the most obvious chance of success may not always be the one that delivers the best result.

If you are looking to compare cameras with different resolutions, you should be downsampling to the lowest common denominator - which is not the same as cropping. Downsampling (aka resampling to a lower resolution), preserves most of the detail of the hi res capture, and it will average out the noise among pixels - to provide a good basis for comparison.

Here is a technical description of how this happens, and it discusses 4 types of noise that are mitigated when downsampling

https://photographylife.com/why-downsampling-an-image-reduces-noise

If you are printing, there is no need to consider the D500 because it just can't compete with the other cameras you have mentioned - especially if you want to compare uncropped images. The reason being that an uncropped image from a D500 would have to be magnified considerably more to achieve the same print size - it is not so much about pixel density. But a 45 mp image resampled to 20.6 mp would likely be no noisier, and likely record more detail that will still be visible in the downsampled image.

As far as mirrorless cameras are concerned, the hybrid, high performance focusing systems are better than even what is in the D6 or the D850, and all three mfgrs - Sony Nikon and Canon have competing systems - and they are desirable over the DSLR - which, by comparison, is not as consistent and not as accurate due to the nature of phase detect AF systems.

Ha! This topic is pretty well beaten to a pulp, but I have to add my tuppence because just last week I bought a D5 (with less than 3K clicks) and have been shooting and comparing it to my other three cameras: the Z7, Z6 II and the D850. So far, what I can see from real world shooting of backyard wildlife in low light, in trees, and on the wing, is the D5 is superior to the others, not so much for the low-light aspect of noise, although the D5 does seem to be better (see image attached); they are all pretty close in that respect and the noise can be mitigated in post. What's more amazing is the ability of the D5 to grab focus almost instantly and hold that focus on distant subjects, with the two long lenses I've been shooting with (Nikon 500mm PF and Sigma 60-600mm S). Shooting a bird in a tree behind or through branches, the D850 is slow and hunts, the Z6 II fails and the bird is gone, and I don't even bother trying those same shots with the Z7.

The other unexpected beauty of the D5 is the incredible sharpness of the photos I've been getting with the it. It really reminds me of the D500: the camera performs much better than the specs say it should. The D5 images are easily equal to the D850 especially when processed with DxO PureRAW 2 and Enhanced in Lightroom to double the resolution. But again, it's the D5's focusing and tracking ability that wins me over. The D850 is good, the D5 is better.

As for the FPS, I don't like blasting off 120 frames in 10 seconds and have to deal with hundreds of images to sort through and rate or delete—it's a waste of time. Most birds in flight aren't going to be around much longer than 15-20 frames, and static subjects, well what's the point of shooting 12 fps when something is barely moving?

All that said, I do have a Z9 on order and if it ever gets here I'll have a heck of time deciding which one of the other cameras to keep as backup, because the IQ differences between the Z7, Z6 II, D850 and D5 are not a criteria for making that decision. And I really don't need 5 great cameras.

In case anyone is wondering, the workflow I used on the attached image is this: Make exposure adjustments in Lightroom Develop, and Crop > Enhance in Lightroom w/o Super Resolution > Process in DxO PureRAW 2 (an amazing plugin/app) > Enhance again in Lightroom for Super Resolution > Edit in Photoshop for final little tweaks and reduce for Web using Steve Perry's action for doing that.