Sauté Aromatics: In a large skillet, melt butter and reserved bacon grease over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook until fragrant, about 1 minute more.

Combine Ingredients: Transfer the onion and garlic mixture to a 6-quart or larger slow cooker. Add the cubed potatoes and chicken broth. Season with salt, pepper, and optional garlic powder, onion powder, and cayenne pepper.

Slow Cook: Cover and cook on low for 6-8 hours, or on high for 3-4 hours, or until the potatoes are very tender and easily mashed with a fork.

Blend (Optional): For a smoother soup, use an immersion blender to partially blend the soup. Be careful not to over-blend, as this can make the soup gummy. Alternatively, carefully transfer half of the soup to a regular blender and blend until smooth, then return it to the slow cooker.

Add Dairy: Turn off the slow cooker or switch to the "warm" setting. Add the softened cream cheese, sour cream, and milk. Stir until the cream cheese is completely melted and the soup is smooth.

Stir in Cheese and Bacon: Stir in 1 1/2 cups of the shredded cheddar cheese and most of the crumbled bacon, reserving some for garnish.

Season and Serve: Taste and adjust seasonings as needed. Ladle into bowls and garnish with remaining cheddar cheese, bacon, and fresh chives.