I always use a gimbal head for that lens and a ball head for everything else. Doesn't make it a lot of fun to switch back and forth, though. In that case it's easier to carry two tripods and use Arca-Swiss quick release systems for both.
I've had a D7100 since 2014. I love it. You say you take nature shots, if that includes animals, birds, insects - anything that moves, go for the D7200 because of the larger buffer. However, if it were me, I'd bite the bullet and go for the D500.
I've had the Nikkor 18-300 since it came out in 2012. I bought for my D5100 and use it on my D7100 now. I love it. I don't have to change lenses like I used to. It seemed like I always had the wrong lens on the camera. No longer. The other advantage is that your sensor stays cleaner because you're not always exposing it to dust. Good luck with it.