Add 6 cups broccoli, 1½ cups onion, chopped celery, 1 cup evaporated milk, ½ cup sour cream, ¼ cup water, 2 teaspoons mustard, 1 teaspoon each garlic powder and onion powder, ¾ teaspoon salt, ½ teaspoon pepper, 3¾ cups broth and 1 cup Cheddar to a 6-quart slow cooker; stir to combine. Nestle chicken into the mixture, submerging completely. Cover and cook until an instant-read thermometer inserted in the thickest portion of chicken registers 165°F, about 4 hours on High or 7 hours on Low. Transfer the chicken to a cutting board; shred, using 2 forks.

Whisk 2 tablespoons cornstarch and the remaining ¼ cup broth in a small bowl until smooth. Stir into the soup, along with cream cheese, 2 cups rice and the shredded chicken. Cover and cook on High until the rice is tender and the soup has thickened, 10 to 15 minutes. Stir to combine.

Divide the soup among 8 shallow bowls. Top with the remaining 1 cup Cheddar, ½ cup scallions and 3 tablespoons bacon.