You didn't mention budget, however, if I were you and what you said as priorites, I'd get the Z6 iii AND the Z8.
I am betting the Z6iii is also much better at higher iso than the z8, much like the R3/1 being noticeably better at high iso than the R5 ii. In fact, the R1 is massively better at high iso than the Z8/Z9 and I've seen it in my own use recently.
Also, as many mentioned, the video stuff, wow, I just read through quickly about the Z6 iii and it's like the canon top video specs. I did not see if the z6iii can do ProRes but that was one reason I wanted the Z9, for internal prores. I very much prefer full-size/"pro" bodies though so that is a big factor for me choosing the Z9 and R3 and R1.
The price point with the current $2100 on the Z6 iii is incredible in my opinion. When you mostly "fill the frame", 24Mp really is "enough". I use the 45Mp bodies so often knowing I'll be cropping to half that.
Here's a shot with the 24Mp canon R1 as iso 25,600 and a 2x tele ext on a 600mm, Handheld at 1/200s in basically light too dark to read with mid 50's eyes ;- )
I know it's the canon but it's still "only" 24Mp but in low light, low Mp wins.
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