Thoroughly lubricate the interior of a 6-quart slow cooker with non-stick cooking spray or a thin layer of unsalted butter to prevent protein adhesion.

In a skillet over medium-high heat, sauté the ground sausage until reaching an internal temperature of 160°F (71°C). Remove from heat and drain all rendered fat through a fine-mesh sieve.

Distribute 50% of the frozen hash browns into a uniform layer at the base of the slow cooker.

Incorporate half of the cooked sausage, half of the diced onions, and half of the diced bell peppers as the second layer.

Apply a 1-cup layer of shredded cheddar cheese over the vegetable and meat stratum.

Repeat the layering process once more with the remaining hash browns, sausage, onions, and bell peppers (reserving the final cup of cheese for the terminal cooking phase).

In a high-capacity mixing bowl, whisk the eggs, whole milk, salt, black pepper, and garlic powder until the mixture achieves a homogenous, aerodynamic consistency with no visible streaks of albumen.

Slowly pour the egg mixture over the layered components, ensuring even distribution across the entire surface area for uniform hydration of the hash browns.

Affix the lid and calibrate the slow cooker to 'LOW' heat for a duration of 7 to 8 hours, or 'HIGH' for 3 to 4 hours, until the center is firm and the edges exhibit Maillard browning.

Top with the remaining 1 cup of cheese during the final 15 minutes of the cook cycle to allow for optimal melt and surface tension.

Switch the unit to 'OFF' and allow the casserole to stabilize for 10 minutes before slicing and serving.