I too have the Z6 which I purchased as a replacement for a dying D300 and as backup to my D800.

Technically I can not fault it and have found it handles challenging scenarios such as retaining focus with AF on runners coming towards me well.

During the past few days I have used it with the FTZ, a 1.4 TC and 80-400mm at a couple of locations both on a tripod and off and was stunned with the results of a grab shot of a kingfisher. 580mm, 1/25s, hand held (shoulder supported), a very severe crop and a keeper at 1800 px wide. Was it luck, in camera VR or AF accuracy of mirrorless?

What I can not get used to is the small size of the camera necessitating the closeness of controls on the rear right side. To use the cursor I have to use my thumb nail, which just seems wrong.

The only other annoying thing I have found is that when using the 'Focus Shift' function, once setting up and starting the stack shots the EVF/Monitor is blank. This makes it totally impossible for doing such an obvious procedure as tracking a moving critter until that instant it stops moving then pressing the button and the sequence firing off.

Original full frame and below the edited version with basic capture, creative and output sharpening

I have been photographing wildlife in Florida for the past 8 years every day of the week. I have rented almost every major mirrorless brands including Canon, Nikon, and Sony. I can confidently state that the best focusing system belongs to the Sony a1, followed closely by the Sony a9 and a9II, followed by Canon and then Nikon. My tests included the Nikon Z6II. I guess you could call my tests guessing, but my images speak for themselves. I am happy with my Sony images followed by Canon and then Nikon. I further believe the Nikon Expeed processing engine the best in the market. But for me, I believe the focusing system of the Sony to be the best in the field. And I have always liked Nikon, check out my screen name.
My tests were the furthest thing from scientific, but my Sony images were the best, sharpest, and most consistent of the big 3.

I've shot action, such as barrel racing at very close and far distances, and the Z6ii has performed very well. No complaints.

I've also shot wildlife from squirrels to mountain goats to moose with excellent results. Haven't shot many birds but so far no problems with the ones I did shoot.

Depending on the circumstances it isn't as good as a D500 for action but is better in other ways. It is not as good as a D850 but it's size is significantly smaller and lighter so it is a better choice when packing a camera into the mountains---at least for me.

The limited choice of available long lenses is my major complaint.

That is what my draw was initially to Sony, they brought out the 400 2.8, 100-400, 200-600, and 600 f4. The only thing that Nikon has now in Z long is the 100-400.
And Nikon is brining out a 200-600 but their 400 2.8 will have a 1.4 teleconverter built in causing it to be heavier than their current monster weighted 400 2.8, not the game changer I was hoping for.
I love the Nikon system, but they just keep missing the mark when it comes to competing with Canon and Sony.
So, until Nikon can compete at the long lens level, I am sticking with Sony for my wildlife and birds in flight photography.