Preheat oven to 350°F. Spray a 9x13-inch baking dish with cooking spray and set aside.
Place the 16 oz bread cubes in a large bowl.
In a medium bowl, whisk together 4 eggs, 1 cup almond milk (or regular milk), 1 tsp vanilla extract, 1 tbsp granulated sugar, and 3/4 tsp ground cinnamon until blended.
Pour the egg mixture over the bread cubes, stir to coat, and let sit 5–10 minutes, stirring occasionally to ensure the bread soaks evenly.
In another medium bowl, combine 5 cups frozen berries, 1/2 cup granulated sugar, 1/2 tsp ground cinnamon, and 1 tbsp cornstarch; stir until the sugar and cornstarch are evenly distributed and the berries are lightly coated.
Spread the berry mixture evenly in the prepared baking dish.
Spoon the soaked bread cubes evenly over the berries, pressing gently so the bread makes contact with the berries and any remaining custard drips over.
Sprinkle 1 tbsp granulated sugar evenly over the top of the casserole.
Bake on the center oven rack for 35–45 minutes, or until the top is golden brown and the berry mixture is bubbling.
Remove from the oven and let the casserole rest about 10 minutes before serving.