Season: Combine all rub ingredients. Pat the pork dry and massage the rub all over, pressing it in.
Sear (optional): Brown the pork on all sides in a hot, oiled skillet for deeper flavor.
Layer: Scatter sliced onion in the slow cooker. Set the pork on top and pour the cola around the sides (not over the top).
Cook: Cover and cook on LOW 8–10 hours or HIGH 5–6 hours, until the pork shreds easily (about 200–205°F internal).
Make the sauce: Whisk all sauce ingredients in a saucepan. Simmer over medium heat 8–10 minutes until slightly thickened. Reserve about ⅓ for serving.
Shred: Transfer pork to a bowl, shred with two forks, and discard large fat pieces. Add a ladle of cooking liquid to keep it juicy, then stir in most of the sauce.
Serve: Pile onto toasted buns with slaw, pickles, and a drizzle of reserved sauce.