Pat both sides of the shrimp dry with paper towels. Toss in a large bowl with garlic, salt, cumin, chili powder, onion powder, and 3 tablespoons olive oil. Cover and refrigerate for at least 15 minutes (up to 8 hours). In a large pan, melt butter and remaining 1 tablespoon olive oil.
Cook shrimp in batches to avoid overcrowding the pan. Add shrimp in a single layer and sauté until they just turn pink, flipping halfway, 2 to 4 minutes total depending on their size. Remove from heat and set on a cutting board. Cut off the tails. If desired, add a squeeze of fresh lime juice to the shrimp.
Zest and juice limes to get 1 teaspoon zest and 4 tablespoons juice. In a small blender or mini food processor, combine all the sauce ingredients. Add salt and pepper (or to taste) and blend until smooth. Taste and adjust to preference. Refrigerate until ready to use.
Remove 1/2 cup of cilantro-lime sauce and toss it with the shredded green cabbage to combine.
Optional: Spray tortillas (both sides) with cooking spray and grill on the stovetop over an open flame until slightly charred. Alternatively, warm tortillas in the microwave, covered with a damp paper towel.
Fill tortillas with a generous serving of slaw, shrimp, and fresh avocado. Drizzle more sauce on top and serve with a squeeze of fresh lime juice.
