Make the Beer-Braised Carnitas: In a small bowl, whisk together the salt, smoked paprika, cumin, onion powder, chili powder, garlic powder and black pepper; set aside.
Heat the vegetable oil over medium-high heat in a large cast iron skillet. Sear the pork roast on all sides until golden brown (about 2 minutes per side). Transfer the pork roast to a slow cooker and sprinkle the seasoning mixture evenly all over, then pour the beer over the pork. Cook on low for 10 to 12 hours, turning and rotating the pork roast a couple of times if possible.
Once finished cooking, shred the pork with two forks, then leave in the slow cooker on warm before serving. The carnitas can be made up to three days in advance. Reheat in a covered pot with ½ cup water, adding more water if needed to keep pork moist.
Assemble & Bake the Nachos: Preheat the oven to 350 degrees F. Spray two large rimmed baking sheets with non-stick cooking spray.
Dividing evenly between the baking sheets, layer the tortilla chips, carnitas, black beans, onion and shredded cheese, repeating the layers twice more, gradually building up a mound in the center as you go. Bake for 20 to 25 minutes, or until the cheese is completely melted. Sprinkle with fresh cilantro and serve with desired toppings.