Place the chicken thighs in a 5–7 quart slow cooker in an even layer.
In a small bowl, whisk together chili powder, smoked paprika, garlic powder, cayenne (if using), salt, and freshly ground black pepper.
Pour the spice mixture over the chicken and add the grated yellow onion; toss to coat the chicken evenly with the spices.
Pour 3/4 cup of the BBQ sauce evenly over the chicken.
Cover and cook on low for 4–6 hours, until the chicken shreds easily and registers 165°F (74°C) in the center.
Transfer the chicken to a cutting board and reserve the cooking liquid in the slow cooker; skim and discard any fat from the liquid.
Shred the chicken with two forks to your desired size (slightly larger is fine because it will break up more when mixed).
Return the shredded chicken to the slow cooker. In a microwave-safe bowl, heat the remaining 3/4 cup BBQ sauce with 2 tablespoons steak sauce until warm (about 1–2 minutes).
Pour 3/4 cup of the reserved cooking liquid and the warmed BBQ-steak sauce mixture over the chicken and toss to coat; add another splash of reserved liquid or more BBQ sauce if needed and adjust salt to taste.
Serve the pulled BBQ chicken on hamburger buns.
