Slice calamari tubes into bite-size pieces. Also, chop any larger tentacles. Soak calamari in buttermilk or yogurt solution along with salt and pepper for at least 30 minutes in the refrigerator.  

Meanwhile, pulse mini pretzels and flour together in a food processor until pretzels are finely crushed, but still have some texture. Season with black pepper and red pepper flake if using. Place pretzel-flour mixture in a shallow dish. 

Spray air fryer basket with oil. Preheat air fryer to 360 degrees F. Drain calamari from marinade. Dip calamari into pretzel-flour mixture and roll around until completely coated. Place calamari in air fryer basket and spray or brush with oil. Air fry calamari for 10-12 minutes, turning and brushing or spraying with more oil about halfway through. 

While calamari cooks, mix together equal parts mustard and mayonnaise in whatever amount desired for dipping.

Once calamari is cooked, serve immediately with sauce, lemon slices, and parsley on top.