Preheat oven to 350°F. Spray a 9x13-inch baking dish with cooking spray.
In a large bowl, whisk together eggs, milk, garlic powder, salt, and pepper until well combined.
Stir in hash browns, bell peppers, spinach, zucchini, mushrooms, and 2 cups of shredded cheese. Mix until everything is evenly distributed.
Pour mixture into prepared baking dish and spread evenly. Top with remaining 1 cup of cheese.
Bake for 40-45 minutes until eggs are set and top is golden brown. A knife inserted in the center should come out clean.
Let cool for 10 minutes before serving. To freeze: cool completely, cut into portions, wrap individually in plastic wrap, then place in a freezer bag for up to 3 months. Reheat in microwave for 2-3 minutes.