Plug in your 6 to 8-quart (5.7 to 7.6-liter) slow cooker and let it sit on your counter. No need to preheat.
Place your 1.5 pounds (680 grams) of chicken breasts at the bottom of your crockpot, laying them relatively flat and side by side.
Add the rinsed black beans, drained corn, undrained diced tomatoes, undrained Rotel, chicken broth, taco seasoning, ground cumin, and chili powder directly into the crockpot.
Using a large wooden spoon or ladle, gently stir everything together until well combined. Make sure the spices are distributed and the chicken is at least partially submerged.
Place the lid on your crockpot and select the LOW setting for 6 to 8 hours (or HIGH for 3 to 4 hours if short on time).
After cooking time, carefully remove one chicken breast and pierce it with a fork. It should shred easily with minimal resistance.
Carefully remove all the chicken breasts from the crockpot using a slotted spoon or kitchen tongs and place them on a clean cutting board.
Using two forks (one in each hand), pull the chicken apart into bite-sized shreds. Hold one fork steady and use the other to pull apart the meat.
Scrape all the shredded chicken back into the crockpot with the soup. Give everything a gentle stir to redistribute the chicken throughout.
Ladle a small taste of soup into a spoon and try it. Adjust seasonings as needed by adding salt, cayenne pepper, or a squeeze of fresh lime juice for brightness.
Use a ladle to pour the soup into individual bowls, making sure each bowl gets plenty of broth and chicken pieces.
Set up your toppings station with crushed tortilla chips, shredded cheese, sour cream, and sliced avocado. Garnish each bowl with fresh cilantro if desired.