Place the salt, garlic powder, onion powder, smoked paprika, and black pepper in a small bowl.
Stir to combine the seasoning blend.
Drizzle the pork with olive oil.
Sprinkle the pork with half of the seasoning blend. Set the other seasoning portion aside.
Using your hands, generously rub the pork with the oil and seasoning.
Preheat a grill or grill pan over medium-high heat.
Once hot, add the pork, and grill it on all sides until it’s seared and grill marks have formed, about 3 minutes per side.
Transfer the grilled pork to a slow cooker.
Add the beer and remaining seasoning blend to the slow cooker.
Cover and cook on low for 12 hours or high for 6 hours or until fork-tender.
Remove the pork from the slow cooker and shred it into bite-sized pieces with a couple of forks.
Remove ⅔ cup of pan drippings from the slow cooker and set it aside.
Return the shredded pork to the slow cooker, stir to combine with the remaining pan drippings, and keep warm until ready to serve.
To make the barbecue sauce, place the reserved pan drippings in a small saucepan and add the tomato paste, Worcestershire sauce, vinegar, mustard, brown sugar, garlic, onion, chili powder, coriander, and celery salt.
Whisk to combine and bring to a low simmer over medium-low heat.
Cook the barbecue sauce for 10 minutes, whisking frequently.
Brush the rolls with the softened butter.
Toast the rolls until slightly crisp and golden brown.
Divide the pork between the toasted rolls.
Serve the pulled pork sandwiches with the barbecue sauce, pickles, and pickled onions on the side for sandwich customization.