Add chicken to slow cooker and cover with barbecue sauce. Toss once to coat. Cover and cook on low for 6-7 hours or high for 3-4, until chicken is cooked through and falls apart easily.

Remove chicken from slow cooker and shred using two forks. Return to slow cooker and toss with juices. Cover and keep warm until ready to use.

Meanwhile, line a large baking sheet with parchment paper and preheat oven to 425 degrees.

Slice sweet potatoes into 1/4" thick rounds. Toss with 1 tbsp olive oil and sprinkle with chili powder. Spread sweet potatoes in an even layer on prepared baking sheet, being careful not to overcrowd the pan (you may need to use two baking sheets).

Bake sweet potatoes for 30 minutes, flipping halfway through. At the halfway mark (15 minutes), toss corn with remaining 1 tsp olive oil and add to baking sheet. Return sweet potatoes and corn to oven for the final 15 minutes, or until sweet potatoes are crisp and corn is lightly roasted.

Remove corn from pan and set aside. Top the sweet potatoes with shredded chicken, black beans, and cheese. Return to oven for about 5 minutes, or until cheese is melted.

Top the nachos with the remaining ingredients (tomato, scallion, cilantro, and avocado) and drizzle with additional barbecue sauce.