It doesn’t really take much time to prepare a breakfast if you don’t expect high cuisine. I get up shortly after 6, cook steaks or salmon for the four of us, drink my coffee-eat and am showered, dressed and out the door, kids in tow, by 7.
But to answer your question, when we ate cereal we would buy a big box of oats from Costco, it lasts a millennia, and mix nuts, berries etc for variety. Tasty congee is easy to prepare and keeps well in the fridge. You can add all kinds of different ingredients to that.