Cook the pasta: Bring a large pot of salted water to a boil.
Add macaroni and cook until just past al dente, about 1 minute longer than package directions. Slightly softer pasta absorbs dressing better.
Rinse and cool: Drain the pasta and rinse under cold water until cool. Shake off excess water well so the salad won’t turn watery.
Whisk the dressing: In a large bowl, whisk together mayonnaise, sour cream, Dijon, vinegar, sugar, celery salt, kosher salt, and black pepper until smooth.
Fold in the mix-ins: Add celery, red bell pepper, red onion, and relish.
Stir to coat with the dressing.
Add pasta and eggs: Fold in the cooled pasta and the chopped eggs, if using. Mix gently until everything is evenly coated.
Taste and adjust: Add a pinch more salt, pepper, or sugar if needed. If it feels too thick, splash in a teaspoon of milk or a bit more vinegar for brightness.
Chill: Cover and refrigerate for at least 1 hour.
The flavors meld and the pasta absorbs some dressing.
Finish and serve: Right before serving, stir and check the texture. If it looks dry, add a spoonful of mayo or a splash of milk. Top with fresh parsley or dill.