Consider oat groats. The whole oat seed. Takes forever to cook, but much lower glycemic index, highly filling, and very, very tasty. They’ll still have some carbs, but have a ton of fiber, germ, and endosperm. Yum, endosperm!
Otherwise I believe both quinoa and wild rice have more protein and fewer carbs than standard processed oatmeals. We use both in our household.
Popcorn’s pretty low in carbs too, it seems
Here’s a list comparing carb loads of common grains. Unprocessed oats are lower than most items on the list.
Also, You can pressure cook your groats in 30 minutes or so. Sometimes we make a few day’s worth that way.