Place the sliced onion rings in a baking dish and pour the buttermilk over them so they are fully submerged; cover and refrigerate at least 2 hours or up to overnight.
In a shallow bowl, whisk together 1 1/2 cups flour, 1 teaspoon salt, 1 teaspoon pepper, 1/2 teaspoon garlic powder, and 1/2 teaspoon smoked paprika to make the dredge.
Heat a few inches of vegetable oil in a large saucepan or Dutch oven to 350°F (use a candy thermometer to check).
Remove onions from buttermilk, shake off excess, dredge rings in the seasoned flour, and fry in hot oil until golden and crispy, about 1–2 minutes per batch; transfer to paper towels to drain.
For the queso, melt 1/2 tablespoon butter in a small saucepan over medium heat, whisk in 1/2 tablespoon flour and cook 1–2 minutes to form a roux.
Slowly whisk in 1/3 cup half-and-half and cook until slightly thickened, about 1 minute, then reduce heat to low and stir in 4 ounces grated cheddar until smooth; season with salt and pepper and keep warm.
Preheat the oven to 400°F.
Toss the shredded chicken with 2/3 cup BBQ sauce.
Arrange tortilla chips on a baking sheet, sprinkle with half of the grated cheddar, top with the BBQ chicken and grilled corn kernels, then sprinkle with the remaining cheddar.
Bake the nachos 10–15 minutes until the cheese is melted and chips are crisp; remove from oven.
Top the baked nachos with sliced green onions, chopped avocado, quartered grape tomatoes, sliced radishes, and torn cilantro.
Arrange the buttermilk onion rings on top, drizzle the warm queso over everything, and finish with extra BBQ sauce if desired.