For salsa, place first 5 ingredients in a small bowl; stir in 1 teaspoon cumin and 1/4 teaspoon salt. Let stand until serving.
Preheat air fryer to 400°. Sprinkle steak with the remaining cumin and salt. Place on greased tray in air-fryer basket. Cook until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°), 6-8 minutes per side. Remove from basket and let stand 5 minutes.
Meanwhile, place onion on tray in air-fryer basket. Cook until crisp-tender, 2-3 minutes, stirring once. Slice steak thinly across the grain; serve in tortillas with onion and salsa. If desired, serve with avocado and lime wedges.