I have a similar set up to what you are thinking. Di2 sprint button to XTR rear. I am currently running 40x11-46 and I wouldn't want to go higher than 42T in the front, I almost wish I did 38T, but at the moment I am happy with the 40T. The 11-46 shimano cassette goes from a 37 to a 46 so the last gear is really just to granny it up the mountain.
I don't have experience with flared bars but some people swear by em.