Mix the dry rub: combine brown sugar, smoked paprika, cumin, chili powder, garlic powder, onion powder, salt, black pepper, and cayenne pepper if using.
Pat the pork shoulder dry and rub the spice mix all over. Wrap and refrigerate for 4-12 hours for best flavor.
Place sliced onion in the slow cooker bottom; set pork on top, fat side up. Pour apple cider vinegar and beer or apple juice around the pork.
Cover and cook on low for 8-10 hours or high for 5-6 hours until pork is fork tender.
Remove pork from slow cooker and let rest 10 minutes before shredding with forks.
Skim fat from cooking liquid, then return shredded pork to slow cooker with BBQ sauce and splash of cooking liquid. Mix and keep warm.
Make coleslaw by combining coleslaw mix, mayonnaise, apple cider vinegar, sugar, salt, and black pepper. Refrigerate at least 30 minutes.Slice Hawaiian rolls in half. Layer pulled pork, drizzle extra BBQ sauce, and top with coleslaw. Add optional toppings like pickled onions, cheese, or pineapple tidbits if desired.
For an optional finish, place assembled sliders on a baking tray, brush tops with melted butter mixed with garlic and brown sugar, cover loosely with foil, and bake at 350°F for 15-20 minutes until warm and golden.