Ditto, oatmeal. It's often my pre-ride meal, even at night if I'm hungry. Oatmeal, sliced banana, some unsweetened Greek yogurt, mixed in a bowl with some spices -- usually cinnamon and turmeric. If I add sweetener it'll be a teaspoon of brown sugar.
I experiment with different types, but usually stick with the convenience of the quick-cook microwavable types. Presumably these are just pre-cooked a bit. But some are close to steel cut rolled oats in texture and flavor without the 20 minute cook times.
I don't feel blood sugar bonks as an adult, but I've noticed a connection between certain types of breakfasts and feeling bonky. The worst for me are pancakes and waffles. I've suspected it might be the syrup, so I've skipped that and tried something like stevia, or nothing as a sweetener. Not sure I notice any difference. Something about eating just pancakes or waffles on an empty stomach gives me a rebound effect of blood sugar rush/bonk. So I'll eat those only with a full breakfast including eggs, bacon and/or sausage, etc.