As already pointed out, shooting with high ISO requires to nail the exposure as brightening underexposed images dramatically increases noise.
Filling the frame with the subject as much as possible is the other key point as cropping will make noise and lack of details more obvious.
I may have missed it, but do you shot jpg or RAW, and what software do you use to process your images.
RAW offers additional gain when a lot of postprocessing is required, and current software can work "miracles" with its noise reduction and sharpening algorithms, even more so when AI steps in (but this requires computer hardware with a lot of horsepower).
At the end of the day we must simply accept that there are limits and that top shots we see are often the result of lots of work and patience in the field (with a good dose of luck) and expert postprocessing.
I would focus for the time being on improving fieldcraft and photography skills along with postprocessing to max out the potential of your current gear.
Realize I'm very late to this thread, but my quick thoughts are:
- the D500 is still an outstanding camera. Are there more advanced, newer cameras? Of course, but they will cost an additional investment and, at least for me, there are still people using the D500 who take better pictures than I ever will on any gear. I still intend to pick one up someday
- Faster/better/longer lenses are always a huge key the puzzle, a benefit of the D500 is the ability to pick up a wide range of F mount lenses used at a discount
- And, as you say (this is a great point you make), post processing can make or break an image. Before investing in a new camera body and new lenses, I always tell people to really break down what they are doing with their processing.
for example, Lightroom + Topaz can cost money, but it's a lot less than buying a Nikkor 500mm f/4E FL ED VR, and Topaz AI (or a similar product) will help a ton on that noise the OP was getting, but for that I think he also needs to follow the other advice in the thread and test/use Auto-ISO and manual to find his tolerance level for noise (or at least what he can process to satisfaction)
And FWIW, I pretty much always shoot in Manual with Auto-ISO, but I have a strange cobbled together kit