Place the red potatoes in a large pot and cover with cold water. Add 1 tablespoon salt to the water. Bring to a boil over medium-high heat.
Cook potatoes until fork-tender but still firm, about 15-20 minutes depending on size. Be careful not to overcook.
Drain potatoes thoroughly and let cool just until they can be handled comfortably.
While potatoes are cooling slightly, prepare the dressing by whisking together mayonnaise, apple cider vinegar, mustard, sugar, pickle relish (if using), salt, and pepper in a large bowl until smooth.
Cut the potatoes into evenly sized ¾-inch cubes and set aside.
Add diced celery, red onion, chopped eggs, and celery seed (if using) to the dressing and gently stir to combine.
Add the potato cubes to the bowl and fold gently with a rubber spatula until all potatoes are evenly coated with dressing. Be careful not to break the potato pieces.
Cover the bowl and refrigerate for at least 30 minutes, though 2-4 hours is better to allow flavors to develop.
Before serving, taste and adjust seasonings if needed. Transfer to a serving bowl and garnish with fresh dill sprigs and a light dusting of paprika if desired.
