After black's ...Bb6, d3 does not look that bad. I personally prefer not to have a bishop on e2 which cannot go anywhere except d1, but maybe in this concrete case d3 is OK because it prevents Ne4 and aids c2-c4.
IMO our white bishop needed to go to f3 (the best diagonal given the Q on a8 and the fact that we are going to attack black's center with c2-c4). Second best would be c2, where it aims at the k-side, which is always useful
So basically I agree that we need c2-c4, but not quite sure that d3 is the best way to do it.
There is nothing wrong with c4 now, but by preparing it with d3 first, it leads into a different type of middlegame. It's a matter of preference as to which type of middlegame one prefers.