I'm a fan of eggs for breakfast and would experiment to see how your LO likes them. (scrambled, boiled, deviled, fried, fried in a hole in the bread, cooked really thin and rolled up with a little cheese or jam... I dunno. My boy likes them scrambled softish with sea salt or ketchup.) Fast, cheap, nutritious.
cottage cheese, cheese quesadillas, yogurt, nut or seed butters, a little mason jar with nuts and raisins to eat on the way to school, homemade muffins (can make ahead, and do very lightly sweetened and high-protein by using recipes for coconut and/or almond flour which use a lot more eggs than normal muffins, adding chopped nuts and seeds). Meat if you eat that (we don't).
Personally we stay away from cold cereal or toast except as more of an occasional treat/snack. My guy is super active and has tons of energy and so I hear you on the kids doing much better with "real" food in the morning. He usually has steel-cut oatmeal with flaxseed meal, topped with a smidge of maple syrup and whole milk, an egg, and fresh seasonal fruit. But, he has plenty of time to eat (early riser).