Place the chicken in the slow cooker and coat with BBQ sauce. Add water if needed. Cook on high for 2.5 to 3 hours, shred, and keep warm.
Marinate chicken in BBQ sauce for an hour, then grill on medium-high for 6-8 minutes per side until cooked through. Let rest before slicing.
Combine quinoa and water in a medium pot. Bring to a boil, then reduce heat and cover. Simmer for 15 minutes, fluff with a fork, and let steam off heat.
Chop the tomatoes, red onion, jalapeño, and avocado. Shred the red cabbage.
Prepare the Greek yogurt ranch dressing or use a store-bought version.
Layer quinoa, shredded chicken, black beans, cabbage, corn, onion, tomatoes, jalapeño, and avocado in bowls. Drizzle with ranch dressing and garnish.