Add the diced onion, chopped jalapeños, and minced garlic to the bottom of the slow cooker, spreading them into an even layer. Place the chicken on top, then season with the cumin, taco seasoning, and a generous pinch of kosher salt and black pepper.
Pour the enchilada sauce over the chicken, followed by 3 cups of chicken broth, ensuring the meat is mostly submerged. Cover the slow cooker with the lid and cook on Low for 6–7 hours or High for 4–6 hours, until the chicken is tender enough to shred easily.
Remove the chicken from the slow cooker and shred it into bite-sized pieces using two forks. Return the shredded chicken to the soup and stir well.
Mix in the cooked rice, chopped cilantro, and freshly squeezed lime juice. If you prefer a thinner consistency, gradually add the remaining chicken broth until the soup reaches your desired texture. Allow everything to warm through for a few minutes before serving.
Ladle the hot soup into serving bowls and garnish with crunchy tortilla chips. Finish with a dollop of yogurt, shredded cheddar cheese, sliced green onions, avocado, mango salsa, and additional cilantro as desired.