Use Table molasses not black-strap molasses.
Honey is in my opinion the king of mellower outers.
Maple syrup is nice too.
Next time change the ratio of sugar to acid. Less vinegar more sugar.
probably a little more apple juice too.
Just tinker with it till you like the taste.
When testing only change one thing at a time or you will be chasing the formula around and getting no where. Change one thing, then taste it then adjust a bit more till that one thing is just right. Then go on to change one other thing. In this way you will hone in on that particular combination of tastes that is right for you.
Definately second the use of mollases and a little more apple juice. I had to try about 3 times till I found a ballance that I liked... but just one more reason to fire up the smoker - Research! (and a beer!)