Cook the bacon until crisp. Set aside on paper towels, then crumble when cool.
In a skillet, brown the ground beef over medium heat. Add onion and cook 3 to 4 minutes. Stir in garlic and cook 30 seconds.
Add chili powder, smoked paprika, cumin, salt, and pepper. Stir in tomato paste and barbecue sauce. Cook 1 to 2 minutes, then remove from heat.
In a bowl, whisk together condensed soup, sour cream, and milk until smooth.
Spray a 6-quart slow cooker with nonstick spray. Add half the potatoes, then half the corn and half the beans. Spoon over half the sauce.
Add the beef mixture. Top with remaining potatoes, corn, and beans. Pour remaining sauce over the top. Stir in most of the bacon crumbles (reserve some for topping).
Cover and cook on LOW for 5 to 6 hours (or HIGH for 3 to 4 hours) until potatoes are fork-tender.
Sprinkle with 1 cup cheddar and all of the Monterey Jack. Cover and cook 15 to 20 minutes, until cheese is melted.
Let rest 10 minutes. Top with remaining cheddar, reserved bacon, and green onions if using. Serve warm.