Season the salmon fillet with garlic powder, paprika, 1 teaspoon salt, and ½ teaspoon black pepper. Drizzle with the oil.
Grill the salmon skin side-down for 5 minutes, until the skin crisps. Flip and cook for another 3 minutes, until the flesh is opaque and easily releases from the grates. Flip the salmon back over and remove from grill.
Cool the salmon slightly, then flake into bite-sized pieces. Discard the skin.
Make the salmon salad: Mix the flaked salmon with the cream cheese, sour cream, mayonnaise, celery, green onions, lemon juice, zest, mustard, and hot sauce. Season with ½ teaspoon of salt and ¼ teaspoon of black pepper. Set aside.
Make the herb-celery slaw: Combine the chopped celery, parsley, dill, lemon juice, mayonnaise, salt, and black pepper. Set aside.
Butter the brioche bread slices. Grill for 1-2 minutes per side, until lightly toasted.
Spread the salmon salad onto two slices of the toasted bread. Top each sandwich with 1 tablespoon of the herb-celery slaw, then top each one with the remaining toasted bread slices.
Serve the grilled salmon sandwiches with the rest of the herb-celery slaw on the side, along with potato chips if desired.