Yeah, the D500 is on my radar. Between the aviation and sports photography, I think a crop body is still my best bet. The D500 looks to have the kind of AF performance, speed and low-light quality I need and it would make a good primary body with my D7100 a useful second body. I'm thinking of buying sometime next year, so prices should have nudged down a bit more by then.
Yep, I had expected the dock to be included - and in fact, the hire place do include it with the Sigma 120-300 they stock - and there just wasn't time to organise for the dock to be sent out, or to find one locally. I've had a chat with the rental place about it and they're likely to start including it with future hires, which was a positive move, and I'll see whether I can wangle a freebie at some point in the future because of the issues I had this time.