Preheat grill to medium-high. Thinly slice the steak and add to a large bowl along with the fajita vegetables, oil, chili powder, smoked paprika, and cumin. Season with salt and pepper. Toss to combine.
Distribute steak and vegetables among 4 (12x12-inch) sheets of heavy-duty foil. Fold sides of foil up around mixture. Roll tightly to seal. Grill packet for 4–5 min. per side.
Carefully open packets. Serve beef and vegetables in the tortillas, topped with guacamole and desired toppings.