Well, 1-1.5 stops less DR than the industry leaders ie D850 and Z7 is no problem IME. The combination of very reliable AF and low light performance at one's fingertips with custom settings has seen the D5 capture me a string of keepers, in fact images I rate as 'lifers'.
I would agree Nikon could have considered a Z6 sensor in the D6 (perhaps they did who knows)...... but time has moved onward. And as with the D5's
out in the wild, the D6 works even better for many pros in wildlife, including challenging conditions.
A useful snippet of advice by Thom H in
his review (more details in his D5 ebook) is dial in +0.3EV, which I have set in Photo Shooting menu, and strive for ETTR. This works AND anyways is only relevant < ISO 800, where intermediate settings are best avoided. But always ETTR. Far far more critical is to avoid cropping!