I've had similar thoughts. I do love the Z6 and it's ergonomics/feel are perfect for me, but my only issues are lack of native 1.4 lenses and focus tracking. Focus tracking is good, not great, and I get more out of focus shots than I feel like I should. Maybe it's user error and maybe my subject matter is too fast (my toddler rarely sits still and is super fast).
I despise the Sony menu system and don't care for the ergonomics (I've owned both the A7R and A7RII), but have toyed with the idea of getting another if it would give better AF tracking. I also discovered recently I qualify for a hefty EDU discount on Sony gear, which I do not on Nikon. I may rent an A9II or A7RIV and 85mm 1.4 in the near future and see how it performs.