I make a very similar recipe using white whole wheat flour and it comes out GREAT!
I know that when I have tried to make only 1/2 of the recipe, it is much harder to get it right.
Have you considered making the whole recipe and freezing the extra challah just barely done?
That is what I do and then I put the challah in the oven to warm up before Shabbos.
DELICIOUS!!!!!!!!!!!