Preheat oven to 425°F. Combine flour, cornmeal, sugar, baking powder, baking soda, salt, and pepper in a large bowl. Using your hands, work butter into flour until pieces are chickpea-size. Add buttermilk and mix just to blend. Drop dough by heaping 1/4-cupfuls into an 8″ cast-iron skillet, spacing about 1″ apart. Bake, rotating skillet once, until biscuits are puffed, golden brown, and cooked through, 1215 minutes. While biscuits are baking, heat oil in a medium skillet over medium. Add chorizo, breaking up any large pieces with a spatula. Cook, stirring occasionally, until chorizo is browned and crisp, 810 minutes. Transfer to a small bowl with a slotted spoon. Whisk flour into drippings in skillet and cook, whisking constantly, until roux is very smooth and starting to turn a light golden brown, about 5 minutes. Gradually add milk, whisking constantly until incorporated. Reduce heat to medium-low and cook gravy, whisking constantly, until thickened, 58 minutes. Stir half of chorizo into gravy; season with salt, pepper, and hot sauce. Spoon some gravy over hot biscuits and top with avocado, scallions, radishes, cilantro, Cotija (if using), remaining chorizo, and more hot sauce. Serve remaining gravy alongside.