Heat oven to 350°F. Lightly grease a 9×13-inch baking dish.
Boil pasta in salted water until just al dente. Drain well.
Cook bacon until crisp, then crumble. Set aside a small portion for topping.
Whisk cream cheese, sour cream, milk, and condensed soup until smooth. Stir in ranch seasoning and garlic powder.
Fold in shredded chicken, half the cheddar, most of the bacon, and most of the green onions. Season with salt and pepper.
Add drained pasta and toss until everything is evenly coated.
Spread mixture in the baking dish. Top with remaining cheddar and the reserved bacon.
Bake 20–25 minutes, until bubbly and the cheese is melted.
Rest 5–10 minutes. Sprinkle remaining green onions and serve warm.