Boil the pasta in salted water until just tender, about 8–10 minutes.
Drain, rinse briefly under cold water, then drain very well.
Spread pasta on a sheet pan to cool and dry for 5 minutes.
Whisk mayonnaise, sour cream, Dijon, vinegar, lemon juice, sugar, salt, pepper, garlic powder, paprika, oil, and flakes.
Combine cooled pasta, celery, onion, bell pepper, peas, pickles, and most parsley in a large bowl.
Fold in the dressing until everything is evenly coated.
Chill for at least 30 minutes. Taste and adjust seasoning. Garnish with remaining parsley.