Bring a large pot of salted water to a boil. Cook macaroni according to package instructions until al dente. Drain and rinse under cold water to cool completely.
In a large mixing bowl, whisk together mayonnaise, apple cider vinegar, Dijon mustard, sugar, salt, pepper, and garlic powder until smooth.
Add the cooled macaroni, celery, red onion, red bell pepper, parsley, and pickle relish (if using) to the dressing. Stir until everything is well coated.
Cover and refrigerate for at least 2 hours, or preferably overnight, to allow the flavors to meld.
Before serving, stir the salad and adjust seasoning with additional salt or vinegar if needed.
Notes
For best results, let the salad chill overnight to enhance flavor. You can also add hard-boiled eggs or shredded carrots for extra texture.