Combine chili powder, paprika, cumin, salt, and black pepper in a mixing bowl
Cut chicken thighs into 1-inch cubes, toss with the spice mixture until well-coated
Chill the chicken in a sealed container for 15 minutes to enhance flavor absorption
Whisk together orange juice and white vinegar in a small bowl, set aside
Heat avocado oil in a large skillet over medium heat
Add the chicken to the skillet and cook for 5-7 minutes, stirring occasionally
Add the orange juice-vinegar mixture to the skillet, cover, and simmer for 30 minutes more
Use a fork to shred the chicken into a pileable consistency
Warm tortillas in a pan, oven, or microwave for 30-60 seconds
Spoon shredded chicken onto each tortilla, top with sliced onion, chopped cilantro, and mashed avocado
Serve immediately with additional lime juice or hot sauce if desired