Cook the bacon in a large skillet until crisp. Remove the bacon, drain on paper towels, and crumble it once cool. Leave about 1 tablespoon of bacon fat in the skillet.
Add the ground beef and diced onion to the skillet. Cook over medium heat until the beef is browned and the onion is soft. Drain any excess fat.
Place the browned beef mixture into the slow cooker. Add the uncooked elbow macaroni, beef broth, milk, ketchup, mustard, garlic powder, salt, and black pepper. Stir until everything is combined.
Cover and cook on low for 3 hours, stirring once halfway through to keep the macaroni from sticking.
Stir in 1 1/2 cups of the shredded cheddar cheese until melted. Sprinkle the remaining 1/2 cup cheese and the crumbled bacon over the top. Cover and cook for 15 more minutes until the cheese on top is melted.