Place the chicken thighs in a large baking dish and the sweet potatoes in another. Set aside.
In a small bowl, stir together the brown sugar, paprika, chile powder, garlic powder, 1 teaspoon of the cumin, the coriander, 2 teaspoons salt, and 1 teaspoon pepper.
Sprinkle half of the spice mixture onto the chicken and the other half onto the sweet potatoes. Drizzle each with 2 tablespoons olive oil and toss each to coat in the spice mixture. Let sit for 15 minutes at room temperature.
Meanwhile, in a medium bowl, combine the black beans with the ½ cup cilantro, the lime zest, remaining ½ teaspoon cumin, and ¼ teaspoon salt. Stir to combine.
Preheat a grill or grill pan to medium-high and oil the grates of the grill. Place the sweet potatoes on the grill (in batches, if necessary) and cook, flipping once halfway through, until tender and grill marks have formed on both sides, about 4 minutes per side.
Place the chicken on the grill and cook, flipping once halfway through, until the chicken has an internal temperature of 165ºF and grill marks have formed, about 7 minutes per side. Transfer to a cutting board and let rest for 5 to 10 minutes, then slice across the grain into strips.
Warm up the tortillas by putting them directly on the gas burner of a stove or on the grill pan with the heat on medium. Use tongs to turn and flip them until soft and pliable and slightly charred in parts, about 1 minute per tortilla.
Divide the tortillas among 4 plates. Top each with chicken, sweet potatoes, beans, pickled cabbage, radishes, and cotija cheese. Garnish with cilantro sprigs and a squeeze of lime, if desired.