Combine shrimp, garlic, black pepper, 3 tablespoons of the olive oil, 1 tablespoon of the lemon juice, and 1⁄2 teaspoon of the salt in a medium bowl; toss to coat. Let stand at room temperature until ready to use.

Heat 1 tablespoon of the olive oil in a 10-inch cast-iron skillet over medium-high. Add bell peppers; cook, stirring occasionally, until tender-crisp and charred, about 5 minutes. Sprinkle with 1⁄4 teaspoon of the salt. Transfer to a plate.

Add couscous to skillet; cook over medium-high, stirring constantly, until toasted and fragrant, about 2 minutes. Add stock and remaining 1⁄2 teaspoon salt; bring to a boil. Cover and reduce heat to medium-low; simmer until tender and almost all liquid is absorbed, about 10 minutes.

Uncover skillet, and top couscous with bell peppers and shrimp. Increase heat to medium; cover and cook until shrimp are just cooked through, 5 to 7 minutes. Remove from heat, and drizzle with remaining 2 tablespoons each olive oil and lemon juice. Garnish with parsley, basil, and crushed red pepper. Serve with lemon wedges.