It appears that the Sigma 135mm f/1.8 Art is the best candidate for the ultimate wide-open (w/ razor-sharp focus) lens. Combined with low CA, and the ability to take advantage of the in-body stacking feature of the D850. I can't think of a better option.
Wouldn't be worth much as a true macro lens, but you're more into close-focus.
With 1:5 magnification, this would mean a 180mm (7.01-inch) subject would fill the 36mm D850 frame, which is about ideal for flower photography.
It is so sharp, you could probably frame a 4" subject, crop, and have better resolution than a common macro lens would give you.
It would be interesting for you to compare in-body stacks of this lens on your D850 to manual stacks with the Zeiss 135 Apo.
(Compare both time + results.)