In a large pot, bring 2 quarts (8 cups) of water to a boil and add the Old Bay seasoning. Cook the macaroni in the boiling water until tender, about 6 to 8 minutes. Turn off the heat, then stir in the shrimp and peas, cooking them until the shrimp turn pink and are just done, about 2 to 3 minutes. Drain the mixture in a colander and let it cool for about 5 minutes. Do not rinse.

Transfer the cooled macaroni, shrimp, and peas to a large bowl. Add in the mayonnaise, buttermilk, chopped celery, and red onion, mixing until everything is evenly coated. Grate the lemon zest and squeeze the juice into the bowl, then stir in the chopped herbs. Adjust the seasoning with more lemon juice and Old Bay to taste, making sure the flavors are bright and well-balanced. Garnish with a sprinkle of Old Bay before serving.