Do not use baking soda in place of baking powder. Baking soda will not create crunchy skin.
To make them extra crispy place the chicken thighs under the broiler for a minute or two. Watch carefully, it doesn’t take long for the chicken under the broiler to go from extra crispy to burnt if you walk away from them. You only need a minute or two.
Chicken thighs can be cooked on a wire rack on top of the baking pan. This allows the juices to drip down from the chicken below. I find the chicken is juicier if you don’t use a rack, but slightly crispier if you do use a wire rack. I suggest lining the pan with foil if you use a wire rack for easy cleanup.