Cook the macaroni in a large pot of salted water until al dente (firm to the bite). Drain and rinse under cold water to stop the cooking. Toss with a small drizzle of oil to prevent sticking, then spread the pasta out briefly to cool.
Make the dressing: In a small bowl, whisk together the mayonnaise, white vinegar, sweet relish, mustard, and a pinch of salt and pepper until smooth. Taste and adjust the seasoning — the dressing should be smooth and creamy with a sweet, tangy, well-balanced flavor. If needed, add ½ teaspoon of sugar or a pinch of onion powder for more depth.
While the macaroni cooks, finely dice the celery, red bell pepper, and red onion. Cube the cheddar cheese into small bite-size pieces and set aside.
Once the macaroni has cooled, add it to a large bowl along with the diced vegetables, peas, and cheese. Pour the dressing over the top and toss until everything is well coated. Give the salad a final taste and season with more salt or pepper if needed — start with ¼ teaspoon of each and adjust to your preference. Cover and refrigerate at least 1 hour before serving.
Stir well before serving — the dressing may thicken slightly after chilling. Then garnish with a sprinkle of paprika or a handful of chopped fresh parsley for added color and freshness.