you might have hit the nail right on the head there Kim
"temporarily" full...… its a good observation. A combination of protein and brown is likely to give a "sustaining" full, in my experience

there is a way to add protein powder to porridge , but Ive not tried it, its an Art I believe to get the method non-gloopy.....is it to add the powder to slightly cooled oats ? No, I don't remember, but if you ask a separate question Im sure someone will know, here or in step one list

oats in shake is another way to be less overwhelming as Allison says - bit by bit. Or less oats, a different brown, combinations of things. You'll find what suits you !