Southern Waldorf Salad

2 qt. water
4 tablespoons plus 1/4 tsp. kosher salt, divided
1 pound peeled and deveined raw medium shrimp
8 ounces baby red potatoes, quartered
3 tablespoons Champagne vinegar
2 tablespoons fresh lemon juice
2 teaspoons minced garlic (about 2 garlic cloves)
2 teaspoons minced shallot
2 teaspoons chopped fresh tarragon
2 teaspoons chopped fresh dill
2 teaspoons chopped fresh flat-leaf parsley
1/2 teaspoon celery seeds
1/4 teaspoon mustard seeds
1/4 cup extra-virgin olive oil
3 cups sliced fresh peaches (from about 3 peaches)
4 cups chopped romaine lettuce hearts (from about 4 lettuce hearts)
2 cups thinly sliced watermelon radishes (from about 4 to 6 radishes)

Salt and Pepper Pecans (recipe follows)

1 cup pecan halves
1 tablespoon melted unsalted butter
1/2 teaspoon kosher salt
1/4 teaspoon black pepper