Bring a large pot of salted water to a rolling boil. Add the elbow macaroni and cook according to package directions until al dente, typically 7-10 minutes. Drain the pasta thoroughly in a colander and rinse with cold water to stop the cooking process and prevent sticking. Set aside to cool completely.

While the pasta cools, in a medium mixing bowl, whisk together the mayonnaise, Dijon mustard, apple cider vinegar, salt, black pepper, garlic powder, and an optional pinch of sugar until smooth and well combined. Taste and adjust seasonings as needed.

In a large mixing bowl, combine the cooled pasta, drained and flaked tuna, finely diced celery, finely diced red onion, and drained sweet pickle relish.

Pour the creamy dressing over the pasta and tuna mixture. Gently toss everything together until all ingredients are evenly coated with the dressing. Be careful not to mash the pasta.

Cover the bowl and refrigerate the tuna pasta salad for at least 1 hour before serving. This allows the flavors to meld and the salad to chill thoroughly. Garnish with fresh chopped parsley, if desired, just before serving.