Preheat oven to 375°F. Grease a 9x13-inch baking dish.
Cover potatoes with cold salted water. Boil, then simmer until tender, 12–15 minutes. Drain well.
Combine beef, breadcrumbs, 0.33 cup milk, ketchup, Worcestershire, onion, garlic powder, salt, pepper, and paprika.
Mix gently until just combined. Press mixture evenly into the prepared dish.
Bake the meatloaf layer for 20 minutes, until just set around the edges.
Mash drained potatoes with butter, 0.5 cup milk, sour cream, remaining salt, and remaining pepper.
Spread mashed potatoes over the par-baked meatloaf. Sprinkle half the cheddar on top.
Bake 20–25 minutes, until hot and bubbling at the edges.
Cook bacon if needed and crumble. Keep warm.
Top with remaining cheddar and bacon. Bake 5 minutes more or broil 1–2 minutes to brown.
Rest 10 minutes. Garnish with green onions and parsley. Add barbecue drizzle if desired.