I would never knock the D500 its the best, as for tracking i never use it, center point only or a small group of points.Mike been watching your BIF for years. I'm an Oly guy who switched over to Nikon because Oly just didn't cut it with BIF. I bought a d500 and its a winner. I also own a z6 and was disappointed with it AF-C-----to be honest the tracking on the z series sucks. Its a great cam except for the shoddy AF system. It has great image quality, light weight, handles nicely,super sharp image with my 24-70 f/4, can use old glass with adapter------but for BIF ----UGGGGGGGGGG. Jus my opinion!!!
Just a little short mate Late next year I'm thinking about a Sony 200-600 with an A6400. Later on I can always add an A9 to replace the A7. AF might be the issue with me though and m4/3 is always here to stay. The crop factor of m4/3 really suits.
I'm inclined to go FX with the Z6 or D750, just to have more separation between m4/3 and the next camera up.Even though we have switched to Olympus from Nikon, we still have a D2X (love the old buddy) and D750 cameras. We still have several Nikon lenses and I almost bit on the trade-in for the Z6. Trade-in value + trade-in bonus and a free Nikon lens adapter which would have brought the purchase price to $1195. I resisted but have had tremors of regret. I loved my Nikons but they got to heavy for me to lug around after I broke my dominant wrist. The Z50 has some interest and I might bite if Nikon sweetens the pot.
The reasons FX is of no interest to me are, the Z7 is too expensive and with those MP i would struggle to get a sharp image, the Z6 if cropped ( i did have a spell with the D750) would not leave enough MP most of the time.I'm inclined to go FX with the Z6 or D750, just to have more separation between m4/3 and the next camera up.
DX does not seem to be enough of a jump, up from m4/3. BUT, Mike's high ISO images are attractive.
The issue I have with the Z50 is the same that I have with the Nikon DX and Canon EF-S lines.
Once you push out of the DX/EF-S lens line, you are dealing with bigger/heavier FF/FX/EF lenses, especially the long lenses.
I got the FX Nikon 70-200/4, because at the time NO ONE had an equivalent DX lens. I wanted faster than the f/5.6 of the DX lenses.If so, you might as well go directly to the FX camera, because you are using FX lenses anyway.