Heat a large skillet over medium-high heat. Add ground beef, breaking it up with a wooden spoon, and cook for 3-4 minutes before adding diced onion and minced garlic. Continue cooking until beef is completely browned and no pink remains, about 5-7 minutes total, then drain excess grease.
Transfer the browned beef and onion mixture to your slow cooker, scraping any flavorful browned bits from the skillet.
Add drained black beans, undrained pinto beans, drained corn, both cans of tomatoes with juices, tomato sauce, beef broth, and all seasonings (taco seasoning, cumin, chili powder, paprika, salt, and pepper) to the slow cooker.
Stir all ingredients thoroughly to combine, ensuring seasonings are evenly distributed throughout the mixture.
Cover with lid and cook on LOW for 6-8 hours or HIGH for 3-4 hours. Resist lifting the lid during cooking as this releases heat and extends cooking time.
About 30 minutes before serving, remove lid, stir well, and taste the broth. Adjust seasonings as needed, adding more salt or a squeeze of lime juice if desired.
Ladle hot soup into bowls and serve with desired toppings: shredded cheese, sour cream, cilantro, tortilla chips, avocado, and lime wedges.