Agree on Miller as the deep backup potentially. He played some minutes at the 5 2 years ago. With him it's whether he's healthy. I am hoping DeWolf can take some minutes here or at maybe even at the 4.
Jackson as the starter, Efe and Gueye as reserves Miller can provide relief for all these guys but when he is in I think he will primarily shift to the 4 at least on D. On offense Miller might be our best post up player outside of Jackson.
I have watched a few games of Gueye in high school and at least in those games he didn't play inside very much on either end. Offensively he was always in the corners. Defensively he played at the top of a 1-3-1 in one game and when they went man in another he picked up a guard with either Bittle or another forward picking up the other teams center.
Unless he has really developed physically I think Gueye is more likely at both the 4 and 3 when he's ready.