should I supplement w formula?

 Looking for some advice from other moms... my LO is 8.5 months old. She's always been given BM (EBF and now pumped milk when I'm away and nurse when I'm with her). 
She's always been on the leaner side & is slow to gain weight. Developmentally she's great - meeting milestones etc. & is pretty active- always army crawling everywhere. 
I know that my breastmilk is pretty "thin" and not creamy which is probably why she's still skinny. She also still wakes up 1-2x at night to nurse. 
I want to give her a bottle of formula before putting her down in hopes of keeping her fuller and also so that she puts on a bit more weight. What do you guys think of this idea? 
Sorry for the long post!