the best animals sightings are early and late when there's little light. of course FF has the advantage in low light. i'd say my 80d is acceptable at iso 3200 but previous crop sensors I have used were pretty lousy at and after iso 1600.
imo, 400mm is plenty of reach. of course there are times you'll wish you had more reach but that would be the case no matter what length lens you are using. in fact 300mm on crop would be plenty, imo.
back then I used the 1d mark III + 100-400L I. I also had a 30d, 16-35L II and 50 1.4.
I found myself wanting more light waaay more than more reach.
we were also only allowed 40 lbs of luggage and gear. I also took a lightweight tripod and a monopod. I used the monopod with tilt head for 95% of the shots from the jeep. 90% of my shots were with 100-400.