If you haven't already, try eliminating one of the most common GI irritants in adults: Most dairy products.
My near lifelong GI problems were misdiagnosed by doctors for decades. In the early 1990s one even suggested I might have a problem with gluten -- yeah, the gluten thing goes way back, long before it become trendy to blame gluten for everything even in the absence of celiac disease.
Turned out to be lactose intolerance. I quit drinking milk, gave up ice cream, cheese, etc. Problem cleared up immediately. It doesn't take long for the body to respond favorably to eliminating dairy, if it's the problem -- a few days at most, often almost immediately.
Now I can eat some dairy products if I'm careful: a little yogurt, some butter, a little ice cream, cheese, whey protein powder, etc.
Combining greasy, fatty foods with diary is still asking for trouble, so I have to be wary of really rich, authentic pizza, although genuine mozzarella cheese made from buffalo milk doesn't seem to cause me the same indigestion as the fake American stuff made from cow's milk.
Another factor for me was minimizing variables in my first meal of the day. My breakfast is usually the same, every day: coffee, oatmeal with a little yogurt and banana. Occasionally I'll add a smoothie made from frozen fruit blends, fresh bananas, unsweetened yogurt, a little protein powder, and either soy or rice "almond" milk or a little water and ice to get the desired texture. I can drink it or add it to oatmeal, especially if it's a hot morning and I don't want to eat anything hot before riding.
I'll vary a lot more with other meals, but minimizing variety in breakfast seems to help regulate my digestion. Quicker and easier to get started too.