Preheat oven to 375°F and spray a 9x13-inch baking dish with cooking spray.
Cook breakfast sausage in a large skillet over medium heat until browned, then add diced onion and bell pepper and cook until softened, about 5 minutes.
In a large bowl, whisk together eggs, milk, salt, pepper, and garlic powder, then scramble in a separate skillet until just set but still slightly wet.
Layer 4 tortillas in the bottom of the prepared baking dish, overlapping as needed to cover the bottom.
Spread half of the scrambled eggs over the tortillas, then top with half of the sausage mixture, 1 cup of hash browns, and 1 cup of shredded cheese.
Repeat layers with remaining 4 tortillas, remaining eggs, remaining sausage mixture, remaining hash browns, and top with remaining 2 cups of cheese.
Cover with aluminum foil and bake for 30 minutes, then remove foil and bake for an additional 15 minutes until cheese is bubbly and golden.
Let stand for 10 minutes before cutting into squares and serving.