In a large skillet, cook bacon over medium heat until crispy, about 8 minutes. Transfer to a paper towel-lined plate and let cool, then crumble. In a large pot over medium-high heat, sauté butter and garlic. Add in milk and flour. Whisk constantly, until slightly thick, about 5 minutes, then add potatoes and season generously with salt and pepper. Increase heat to a simmer. Cook potatoes until creamy and tender, 25 minutes. Stir in cheese and scallions until creamy. Serve topped with crumbled bacon and garnish with sour cream, scallions, and cheddar.