Ideally, instead of goal difference, it should be goals difference from real result. In the case of a 2-1, a 2-0 is closer to the real result than 1-0, because you need to change 1 goal only in the first but 2 goals in the second. But I think that's too confusing, specially since it should only work when you also predict the winning team or if it's a draw, a draw. Because imagine someone put 2-2. All in all, I think the goal difference rule whilst not perfect, is the most effective and the rules were written before it all started, so we can theorycraft but should be happy to comply with the way it works.
@Kins, those are all 15/1 ish odds XD