Although I was happy with the results of the 200-500mm combined with the FTZ II adapter on the Z9, after I received my copy of the 180-600mm and taking it for the proverbial 'spin around the block', for me the 180-600mm demonstrated better performance in every regard. The 200-500mm has since been sold. And, thanks for your initial post and the side-by-side comparison. The differences are subtle, and goes to show just how far zoom technology has come, especially in this focal range.Glad you found it useful.
I think the people that will be super happy with this lens are those coming from the 200-500mm who are used to the weight. It's lighter, faster, internal zoom, 90 degree stop to stop zoom, and has the benefits of native Z controls. That particular upgrade is a no brainer other than spending the $$.
On the other hand, I wouldn't sell my 500PF nor 400 4.5. I'd supplement those two with this one for when zoom is needed but certainly don't consider it a replacement.