For photographing the subjects
you are mentioning, really any camera will do the job perfectly, mirror less or DSLR.
Deliberating about the difference between a Z6II and a Z8 i find very strange, unless its purely financially based.
If its not weight or financially limiting it makes absolutely no sense to even consider the Z6II over a Z8, that said the Z6II is a extremely nice camera and very capable for many many but not all things, in the right hands)
The simple answer is if you have used a Z9 or now even a Z8 you wont want to go near any other Z camera PERIOD, the Z range of cameras prior to the Z8 Z9 basically is outdated and in many ways by extreme technical performance obsolete by comparison to the technology and capacity of the Z8 Z9.
Once you experience a Z8 Z9 you more than likely will expand or grow with your wild life photography experience due to the power and permanence capabilities offered in the Z8 Z9, this will be glaringly obvious if you try to grow or diversify using other Z models.
That all said, my self
i have a Z9 D850 and rent a D6 when really things are tough, i enjoy all of them even my old D3X or DF.
If i had to live with one camera in Nikon mirror less and the budget was really tight, there isn't much i couldn't do brilliantly with either the Z6II Z7II and with yes shock, my personal preference for the Z7II because of its resolution. Its not always about FPS and this hyped tracking trending.
What you achieve, 90% comes from YOU,
as Steve says, date the camera marry the glass, the glass will be more important than any camera.
I find i can get almost the same results with any camera if i adapt, its the glass that makes a profound difference for ME.
Below are some Jpeg snaps on a DF D4 also using 2xTCIII on 300 2.8.
In conclusion the Z6 II is excellent, my preference again personally i would use the Z7II because of resolution, but if you could afford it get the Z8 and forget the agony of deciding as there is no comparison, the Z8 trumps things by a long way.
PS
5 - 10 FPS is all you ever really need for 90% of subjects if you know what your doing.
The surfing shots are massively cropped, but killed on the magazine covers
Only an opinion
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