As always, stunning review with great information that helps with decisions for the field, this is what drew me to your videos years ago and great to continue to see you keep contributing to the community in such an informative and professional manner.
Regarding the test itself, to be honest, I was surprised at how well the Nikon 180-600 and Sony 200-600 hold up, while I agree the 600 PF was doing better I did not feel these two consumer zooms were too far off, for those who can't afford the S lenses, this really shows the zooms are viable performers.
I'm still scratching my head trying to figure out the cost justification of the 600PF over the 400 4.5 + TC. Your comparison was helpful and it was clear the 600 PF was the sharpness and bokeh winner over the 400+TC combo (which was to be expected), but was it enough of a difference to invest in considering the fairly large price gap? For me it makes sense to do the 400 f/4.5, Z 1.4 TC but instead of the 600 PF I would think the 800 PF offers more here (for not much more $$$), this combo gives you very good 400mm f/4.5, really good 560mm f/6.3 and outstanding 800mm f/6.3 - with that setup, I'm struggling to see the "need" for the 600 PF.... I realize this is me and my personal preference and clearly others will see the value in the 600 PF even with these others lenses but your review certainly helps to identify strengths and weaknesses.
I must be honest, the biggest surprise (probably shouldn't be) of your review is how amazingly good the 600 f/4 TC lens is!!! Might be worth saving up for the next few years for that lens, wow!