For the cabbage slaw: Add the 1/2 cup cilantro leaves, mayonnaise, garlic, avocado, lime juice, 1/2 teaspoon salt and 1/4 teaspoon pepper to a food processor. Process until smooth. If the dressing is too thick, add water, 2 tablespoons at a time.
In a large bowl, toss the cabbage with the 1/4 cup chopped cilantro, the green onions and the dressing. Let sit for 15 minutes.
For the fish tacos: Mix together the smoked paprika, chili powder, cumin, garlic powder, onion powder and 1 teaspoon salt in a large bowl. Add the fish and toss gently to coat.
Heat a large nonstick skillet over medium-high heat. Add the grapeseed oil and the fish and cook until the seasoning on the fish is blackened, 1 to 2 minutes per side. Transfer the fish to a plate.
Top each tortilla with 2 slices of fish, about 1/4 cup of slaw and a sprinkle of cilantro. Serve with lime wedges.