I have a D750 and I like it but if I were you I would go ahead and get the D810 for what you shoot, and then start collecting some good FX lenses for it. Start with one and learn how to use it before moving on. Happy shooting!
I am interested in this lens and have read comparisons to the Nikon 200-500mm. What made you decide to go with this lens? I really value your opinion! (I shoot with a Nikon D750). (For Gene)
You may have it set to only take pictures when focus has been achieved. You can change it in menu settings. I can look it up later today to tell you which settings to change.