I am a Canon shooter, but frankly a little disillusioned one at this time: I think my brand is falling pretty far behind in terms of lens options, what with the new 500/5.6 Nikon etc.
Thats another issue.
You have a good grasp imo of all the possibilities.
I agree 100% with Peter above. As much FL as can be obtained.
WIth a crop sensor a 100-400 is just about adequate.
And if you seriously mean that your budget could tolerate a D500 and the Nikon zoom, I would sell everything and go for that!