Prep the pork: Trim excess surface fat from the pork shoulder, but leave some for moisture and flavor.
Pat dry with paper towels.
Mix the spice rub: In a small bowl, combine brown sugar, smoked paprika, salt, black pepper, chili powder, cumin, and cayenne if using.
Season generously: Rub the spice mixture all over the pork, pressing it in so it sticks to every surface.
Build the base: Add sliced onion and minced garlic to the bottom of the slow cooker. Pour in apple cider vinegar, chicken broth, mustard, Worcestershire, and 1/2 cup of the BBQ sauce. Stir lightly.
Slow cook: Place the pork on top of the onions.
Cover and cook on LOW for 8–10 hours or HIGH for 5–6 hours, until the meat is fork-tender and shreds easily.
Shred the meat: Transfer the pork to a large bowl. Remove and discard any large pieces of fat. Shred with two forks into bite-size strands.
Reduce the juices (optional but recommended): Skim fat from the cooking liquid.
Either simmer the liquid in a saucepan for 10–15 minutes to thicken, or leave it as is if you prefer more moisture.
Sauce it up: Return the shredded pork to the slow cooker. Stir in the remaining 1/2 cup BBQ sauce and enough reduced cooking liquid to coat the meat without making it soupy. Taste and adjust seasoning with salt, pepper, or vinegar as needed.
Hold warm and serve: Keep on WARM for up to 2 hours.
Serve on buns with coleslaw and extra BBQ sauce, or over rice, baked potatoes, or roasted veggies.