Add Chicken to Slow Cooker: Place 2 pounds chicken breasts into slow cooker.
Combine Sauce Ingredients: In a medium bowl, whisk 1 cup barbecue sauce, ⅓ cup chicken broth, ¼ cup apple cider vinegar, 1 Tablespoon brown sugar, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon smoked paprika, ½ teaspoon salt, and ¼ teaspoon pepper until combined. Pour sauce over chicken in slow cooker.
Cook Chicken: Cover slow cooker and cook on LOW for 4 hours or HIGH for 2-3 hours, until chicken is tender and easy to shred. If chicken isn’t tender after cooking, allow it to cook an extra 30 minutes and check again.
Shred Chicken: Once cooked, use two forks to shred chicken directly in the slow cooker, mixing it into the sauce. If sauce seems too thin, cook uncovered on HIGH for 10-15 minutes to thicken slightly.
Prepare Coleslaw (Optional): While sauce thickens, in a large bowl, whisk ¾ cup mayonnaise, 2 Tablespoons apple cider vinegar, 1 Tablespoon Dijon mustard, 1 teaspoon sugar, ½ teaspoon celery seed, ½ teaspoon coarse Kosher salt, and ¼ teaspoon pepper until smooth. Add 16 ounces tri-color coleslaw mix and toss until coated. Let rest 10-20 minutes to soften slightly; refrigerate until ready to serve.
Serve: Spoon shredded chicken onto hamburger buns (let excess sauce drip off if needed) or into bowls. Optionally, top with coleslaw and pickle slices, then serve.