In a blender, combine the oats, cane sugar, and coconut sugar. Blend on high until the mixture becomes a fine powder, about 30 seconds. In a large bowl, or a stand mixer, add the oat and sugar mixture, buckwheat flour, cassava flour, pumpkin pie spice, cinnamon, baking powder, baking soda, and sea salt. With a spoon or with a mixer on low speed, gently combine the flours and spices.