A few more thoughts.
As Northern Focus says, your choice may depend a lot on what you like to shoot at the long end and the circumstnaces.
Given your 70-200 mm zoom and 1.4x TC, you also have a 98-280 mm lens. Is your zoom of the Nikon 70-200 f2.8s? If so, you have a 98-280 f4 lens with the TC. Which is pretty close at the long end to a 300 mm PF f4. (If you have the 70-200 mm f4 zoom, you still have 98-280 f5.6 with a 1.4x TC.) Of course, the 300 mm PF is quite a bit lighter than a 70-200 mm f2.8 plus a TC. And the 300 mm PF takes the 1.4x TCIII quite well, giving you a nice light weight 420 mm f5.6 lens (and because it is f5.6, AF still works well on a D850 or D500). Before I got my 500 mm PF, I used the 300 mm PF + 1.4x TCIII a lot. After I got my 500 mm PF, I tended to use the 300 mm PF (when I use it, which is not as often now) without the TC.
As to focal length range, if you do a DX crop on images from your D850 (which is pretty close to a D500 image), your 98-280 mm lens has the FOV of a 137-392 mm lens on an FX body. So in some sense, you have 70-392 mm covered.
A long way of arguing that maybe the 500 mm PF would add more for your kit than the 300 mm PF, although both are great lenses. And it depends what you shoot
As to choosing between the 200-500 mm zoom and the 500 mm PF. I have both lenses. Weight, size and cost differences are obvious. I shoot a lot from a canoe or kayak and walking around; so size and weight matter to me. I also shoot a lot of birds, so even with the 200-500 mm lens, I am often at 500 mm. As a result, I use the 500 mm PF a lot more -- taking out the 200-500 mm lens when I need the flexibility of a zoom in a single camera/lens combo. I often prefer to have the 500 mm PF on my D850 or Z7II and a 70-300 AF-P FX lens on my Z6II if I may need shorter focal lengths.
Other differences: The 500 mm PF is a bit better optically, especially at the edges and in the corners (although that may often not matter much with wildlife shots). The 500 mm PF focuses faster, in my experience, than the 200-500 mm zoom, whether on a D850 or ZII body. I think the 500 mm PF balances better on a ZII body than the 200-500 mm zoom. When I want to use the 200-500 mm zoom, I tend to pick my D850 or D500.
Good luck with you choice. All are excellent lenses.