Preheat oven to 375°F and lightly grease a 9x13-inch baking dish.
Cook pasta until just tender according to package directions, then drain well.
Prepare broccoli by steaming briefly if desired; thaw and drain frozen broccoli.
Stir ranch dressing, sour cream, milk, garlic powder, salt, and pepper together until smooth.
Add cooked pasta and chicken and mix until evenly coated.
Fold in broccoli, most of the bacon, and half of the cheddar cheese.
Transfer the mixture to the prepared baking dish and spread evenly.
Top with remaining cheddar and reserved bacon.
Bake uncovered for 20–25 minutes, until hot, bubbling, and the cheese is melted.
Rest for 5 minutes before serving.