Make the coleslaw dressing: in a large bowl whisk together 1/2 cup mayonnaise, 1/3 cup sugar, 1/2 cup low-fat buttermilk, 3 tbsp lemon juice, 1 tbsp white vinegar, 1/2 tsp salt, and 1/8 tsp black pepper until smooth.
Add 16 oz coleslaw mix, 1/2 sliced medium onion, and 1 thinly sliced red bell pepper to the dressing and toss to coat; cover and refrigerate at least 1 hour.
Prepare the sweet BBQ sauce: in a medium saucepan combine 1/2 cup cider vinegar, 3 tbsp brown sugar, 1 cup ketchup, 1 tbsp tomato paste, 2 tbsp Worcestershire sauce, 1 tbsp yellow mustard, 2 tbsp honey, 1 tsp garlic powder, 1 tsp chili powder, 1/2 tsp onion powder, 1/2 tsp cumin, 1/4 tsp salt, 1/4 tsp pepper, and a dash of ground sage; whisk until smooth.
Bring the sauce to a boil over medium-high heat, then reduce to low and simmer, stirring occasionally, about 45 minutes until slightly thickened.
Stir 4 cups shredded cooked chicken into the warm BBQ sauce and heat through until coated and warmed.
Toast the 6 whole wheat hamburger buns in a toaster or oven until lightly browned.
Assemble sandwiches: place about 3/4 cup BBQ shredded chicken on the bottom half of each bun, top with about 1/2 cup coleslaw, then close with the top bun; repeat for remaining sandwiches.