Make the Spice Rub: In a small bowl, mix together the smoked paprika, onion powder, garlic powder, thyme, cumin, black pepper, cayenne, and kosher salt until well combined.
Season the Fish: Pat the mahi-mahi fillets completely dry with a paper towel. Sprinkle the spice rub generously over all sides of the fish, gently pressing it into the flesh.
Prepare the Citrus Salsa: Slice the top and bottom off the orange. Stand it on a cutting board and carefully slice away the peel and white pith. Holding the orange over a medium bowl to catch the juices, carefully cut between the membranes to release the orange segments into the bowl. Add the diced shallot, minced jalapeño, chopped cilantro, and 1 tablespoon of lime juice. Stir gently to combine.
Cook the Fish: Heat the avocado oil in a large cast-iron skillet or grill pan over medium-high heat. Once shimmering, add the seasoned fish. Cook for 4-5 minutes per side, until the fish is opaque throughout and flakes easily with a fork (internal temperature should reach 145°F / 63°C).
Assemble the Tacos: Transfer the cooked fish to a plate. Use two forks to break it into large flakes, then drizzle with the remaining 1 tablespoon of lime juice. Warm the corn tortillas. To serve, layer each tortilla with shredded purple cabbage, a generous portion of the flaked fish, and a spoonful of the citrus salsa. Garnish with a few extra cilantro leaves and serve immediately.