Preheat the oven to 450°F (230°C).
In a small bowl or glass measuring cup, whisk together the oregano, thyme, basil, cayenne, creole seasoning, Cajun seasoning, black pepper, and salt to make the spice mixture.
Warm four tortillas at a time in the microwave for 1–2 minutes so they are pliable, then rub both sides lightly with some of the olive oil and sprinkle about 1/2–3/4 teaspoon of the spice mix on one side of each tortilla; reserve remaining spice mixture.
Form the tortillas into taco shells by placing them over a taco mold or propping them between the inverted cups of a muffin tin; bake 5–7 minutes or until crisp and lightly browned. Repeat with remaining tortillas.
Heat 1 tablespoon of the olive oil in a large heavy skillet over medium-high heat. Add the diced onion and sauté 7–10 minutes until softened. Add the minced garlic and cook about 1 minute until fragrant.
Add the shrimp to the skillet along with about 1 1/2 tablespoons of the reserved spice mixture and sauté about 4 minutes, just until the shrimp begin to turn opaque in the center. Transfer the shrimp to a small bowl.
Add the remaining 1 tablespoon olive oil to the same skillet if needed. Add the thinly sliced red and orange bell peppers and sauté about 5 minutes until slightly soft but still crisp.
Add the fire-roasted diced tomatoes to the peppers and simmer 3–5 minutes, then return the shrimp to the skillet and toss to combine; simmer 1 more minute and adjust seasoning with up to 1 teaspoon more of the spice mixture if desired.
Fill each baked taco shell with the shrimp and pepper mixture and top with shredded cheddar and diced avocado to serve.
