Honestly, I think it's Pace's hair that's throwing
people off a bit rather than his facial features. They probably wouldn't notice such a stark difference if it was a little shorter and darker. As
@DKDetective pointed out, Pace's facial structure isn't all that dissimilar to how Bruce is depicted in the comics (depending on the artist, that is). Moreover, there are a number of physical characteristics that he shares with comic book Bruce, including a longer head shape, straight nose, "bushier" eyebrows, and even the same color of eyes. If Gunn really wants him to look more like the classic version of the character, all he has to do is is darken Pace's hair and maybe cut it a bit shorter to make him look less like the Pattinson version.