I am not 100% sure, and don’t want to mislead you, but Ive heard you really shouldn’t thicken breastmilk with cereal b/c it can break stuff down in your breastmilk. There are some BFing mamas that can probably confirm what I am saying and maybe give you some other ideas. But if it were me and I didn’t know better (LOL) I’d add the cereal right when it was time to eat so that it isn’t sitting in your milk very long breaking it down even more.
I am sorry I can’t be a huge help here.