Whisk together the mayonnaise, apple cider vinegar, granulated sugar, and celery salt in a small bowl until smooth. Pour over the coleslaw mix, toss to coat, season with salt and pepper, then cover and refrigerate while you cook the chicken.
In a medium bowl, whisk together the BBQ sauce, water, apple cider vinegar, brown sugar, garlic powder, onion powder, smoked paprika, salt, and black pepper until fully combined. Taste and adjust seasoning as needed, then set aside.
Pat chicken breasts completely dry with paper towels and season both sides with salt and pepper. Melt butter in a large skillet over medium-high heat, then add the chicken and sear for 2 to 3 minutes per side until lightly golden — it does not need to be cooked through.
Reduce heat to medium-low and pour the BBQ sauce mixture over the chicken, turning to coat both sides. Cover with a lid and simmer on low for 20 to 25 minutes, flipping once halfway, until the internal temperature reads 165°F — add small splashes of water if the sauce reduces too quickly.
Transfer the chicken to a cutting board and rest for 3 to 5 minutes, then shred using two forks into thin, stringy strands. Return the shredded chicken to the skillet and toss with the remaining sauce over low heat for 2 to 3 minutes until fully coated.
Place the split brioche buns cut-side down in a dry skillet over medium heat and toast for 1 to 2 minutes until lightly golden and slightly firm — watch closely as brioche browns quickly.
Pile ½ to ¾ cup of the BBQ pulled chicken onto each toasted bottom bun, then top with a generous scoop of chilled coleslaw, dill pickles, and red onion if using. Press the top bun down and serve immediately.