I am hoping to stop by one of our local stores where I can compare the size/weight/handling of the 500PF and 180-600, and can also rent them as well if I wish. But before I rent them, I do want to try out my 200-500 with the Z6iii first, if only to see how it compares to the D500 for AF speed and acquisition. I think that both lenses are good options and good alternatives to the 200-500, but as my time to get out and shoot is still quite limited, any outlay of cash right now for a lens has really got to wow me over.I often use my 500 PF on the Z9 and D6. It's one of the best telephotos ever made, and the Z Telephotos are as excellent but they definitely haven't eclipsed the 500 PF
I agree with @Rassie and many others who endorse this light prime, especially as you're keeping your DSLRs.
The Z-mount only 400 f4.5S and 600 PF are in the same niche zone:- They pair well with TC's, being so sharp and light with pleasing bokeh... BUT cost more. With the 500 PF, this 400/500/600 triad are Nikon's Commando Kit primes. Unique solutions for wildlife photography and similar genres, especially if the other options are too heavy.
The long awaited 180-600 has replaced the worthy 200-500. It's more versatile but at least 3/4 kg heavier than any of the triad of Commando-Kit primes.
Detailed comparisons of Nikkor Telephotos
For Z system, including a few F Nikkors - synopsis of Thom Hogan's detailed testing to identify the 'Best Telephoto Options in the Z Mount' : https://www.zsystemuser.com/z-mount-lenses/nikkor-lenses/z-mount-lens-articles/best-telephoto-options-in.html Table added:bcgforums.com
--Ken