I belief all breast milk is equally rich in nutrient, trust in yourself.
According to WHO and kellymom.com, and my basic biology knowledge, breast milk, or even formula, is richer (fatter) and more nutrient than congee / 米糊 for sure.
Up to 6 month, taking solid is for supplement and training for sollowing. Also, the solid should be the supplement of iron.
The concept is not to cut milk but put solid as supplement. So see if 4 milk + 1 supplement is okay to start with...