First, heat a large skillet over medium heat. In a bowl, toss the shrimp with olive oil, Cajun seasoning, salt, and pepper until they are fully coated. Cook in the skillet, stirring so they brown evenly, until they turn pink (about 4-6 minutes). Set them aside to maintain their texture.
Preheat your oven to 350°F (177°C). Butter the baking dish lightly. Meanwhile, bring salted water to a boil and cook elbow macaroni until it’s al dente (don’t overcook because it will finish in the oven). Drain it well, then transfer to a large bowl, mixing in the cooked shrimp, crab pieces, and basil.
In a bowl, whisk together whole milk and evaporated milk, then season with paprika, cayenne, salt, and pepper. Taste and adjust seasoning before adding the beaten eggs (you don’t want to over-salt early). After eggs, stir in most of the cheeses (saving some for topping) so that everything blends into a rich, cheesy liquid.
Pour the cheese mixture into the pasta-seafood-basil bowl and fold gently until everything is distributed. Transfer this into the prepared baking dish. Sprinkle the reserved cheeses, extra basil, paprika, black pepper, and Cajun seasoning on top. Bake it for 45-55 minutes, loosely covering with foil after about 35-40 minutes to prevent the top from burning. Once out, let it rest for 25-30 minutes so it firms up before you dig in.