Combine chili powder, smoked paprika, cumin, garlic powder, onion powder, oregano, salt, pepper, and cayenne in a small bowl. Stir until fully blended.
Pat chicken thighs completely dry with paper towels. Drizzle with 1 tablespoon olive oil and rub to coat. Sprinkle spice rub evenly over both sides, pressing to adhere. Let sit 5 minutes.
Heat a cast iron skillet over medium-high heat for 2 minutes. Add remaining 1 tablespoon oil and swirl to coat. Place chicken in a single layer (don't crowd) and cook 5-6 minutes without moving. Flip and cook 4-5 minutes more until internal temperature reaches 165°F (74°C).
Transfer chicken to a cutting board and rest for 5 minutes. Chop into bite-sized pieces or shred with two forks. Taste and adjust salt if needed.
Warm tortillas in a dry skillet over medium heat for 30-45 seconds per side, or directly over a gas flame, or wrapped in a damp paper towel in the microwave. Stack on a plate and cover with a towel to keep warm.
Dice onion, chop cilantro, slice or mash avocado, and cut lime into wedges. Arrange all toppings in small bowls for serving.
Layer two warm tortillas per taco, then top with chopped chicken, onion, cilantro, pico de gallo, avocado, and cotija cheese. Squeeze fresh lime juice over each taco and serve immediately.