Well, given I am also in the position of thinking about a Z6, I've read a lot of comparisons (mainly from wildlife photographers, but also from more general photographers), Opinions vary, but while it doesn't seem to be quite as good as D850, it still seems to be very close.
There is an interesting (and ongoing) comparison between mirrorless for birds in flight here
https://mirrorlesscomparison.com/best/mirrorless-cameras-for-birds-in-flight/
But note the Z6 is only running at 5.5fps, much slower than the other
A year long review by someone who I seem to remember concludes that after the year he is getting rid of his D850,
View: https://www.youtube.com/watch?v=OpJBtYWzUF8
Another Wildlife photographer Morton Hilmer replaced his D5 with a Z6/Z7 and a D850, and now seems to only ever use the Z6 with his 600mm. This is almost certainly linked to ease of switching between stills and video on the Z6, but he still makes his living from still images, so he must be happy enough with the stills AF performance. As an aside during his 6 weeks on Ellelsmere Island, the Z6s stopped working reliably at -30, but worked flawlessly once the temperatures rose to -25
Two other Youtube wildlife photographers only seem to use their z6s for video and are still routinely using D5s, D850s and D500s.
There have been a few reviews like this where people have hung onto their D8xxx for a while and then decided to keep the Z, but equally several that have gone the other way, and some half and half.
Brad Hill gives a good overview
http://www.naturalart.ca/voice/blog.html in his 04 Nov 2019 : Musings Part 2 - The Nikon Z7 which you need to hunt for a bit, so the most relevant text is below (but he has a lot more to say if you go to the link):
"3. The Autofocus (AF) System
This section is probably going to surprise some folks, but I MUCH prefer the Z7 AF system (and how it performs) OVER that of the D850. Keep in mind when you're reading this that I rarely use my Z7 (or in the past my D850) for shooting extreme action...that's what I use a D5 - and to a lesser extent - a D500 for.
What do I like about the Z7 AF system? Several things. First, I simply find it more accurate than that of the D850, especially when dealing with off-centre subjects (e.g., focused on the foreground when shooting a wide-angle landscape shot). Because Nikon refuses to let us know the relative contribution of phase detect autofocus (PDAF) and contrast detect autofocus (CDAF) in each of the focusing area modes, it's impossible for me to guess WHY the AF system of the Z7 seems that much more accurate than that of the D850. But, it's just something I have noticed time and time again in the field. Like in this Harbour Seal shot taken with my Z7 and the Nikon 500mm f5.6E PF where having the eye-region tack-sharp (and the DoF distributed correctly) was so important:"
My conclusions, for what its worth is that it would seem the D850 (I think this is the commonest comparison) just has the edge, for action, but for everyday photography, their doesn't seem to be anything to worry about with the Zs AF. The early reviews of the Zs suffered from the cameras needing set up slightly differently to the DSLRs they were being compared against, and there has been a massive improvement through firmware upgrades.
