Yes on your Flicker collection
Nice to see you ended up with such an impressive collection from Nikon in just a few years ! The results with the D500 and D810 are already great. I wish you'll find the D850 as good or better than your D500 ;-)
Amazing to see that your 800mm is lighter than my old F* 250-600 ! and even lighter than the Sigma, but at a price ;-)
By the way, how does the Sigma 180mm compare against the good old A* 200mm macro ? At least, great results (butineuses) at this size from the D810 and the Sigma.
Too bad there's nothing like this on the Pentax roadmap yet

I'd rather buy a DFA 135mm f/2 first, but ... (nice results too with your 105mm f/1.4!)
Best regards, Fred
PS:
Some prices are a bit exagerated right now, especially the 200mm macro lenses (no equivalent, a real need for some people, and "almost" easy to use), but this FA 600mm seems to be fair especially for a photographer knowing what to expect (lot of time to learn how to use it but some special bokeh and colors to play with). A fortune but well spent for those who can afford it ?