Bring a large pot of well-salted water to a full boil. Cook elbow macaroni 1 to 2 minutes past al dente, about 9 to 10 minutes. Drain immediately and rinse under cold running water for 30 to 60 seconds until fully cool. Shake off excess water.
Whisk together mayonnaise, apple cider vinegar, yellow mustard, sugar, salt, pepper, and garlic powder (if using) in a large bowl until smooth. Taste and adjust seasoning.
Add cooled macaroni to the dressing and stir until every piece is fully coated. The mixture will look wet — this is correct.
Fold in diced celery, red onion, red bell pepper, and chopped hard-boiled eggs. Use a spatula and fold gently so the egg pieces stay chunky.
Taste and adjust salt, vinegar, or sugar as needed. Cover tightly and refrigerate for at least 60 minutes (overnight is better).
Stir before serving. If the salad has thickened too much, fold in 1 to 2 tablespoons of fresh mayonnaise. Garnish with fresh parsley and serve cold.