Place noodles in a large bowl; pour hot water over to cover. Let soak until tender but not mushy, 5-10 minutes. Drain; set aside.

Heat vegetable oil in a wok or large skillet over medium-high heat. Add egg; stir until barely set, about 30 seconds. Add shrimp, if using. Cook, stirring, until shrimp and egg are almost cooked through, 2-3 minutes. Add tofu and radish; cook for 30 seconds. Add noodles and cook for 1 minute. Stir in sprouts. Add tamarind water, fish sauce, and simple syrup and stir-fry until sauce is absorbed by noodles and noodles are well coated, about 1 minute. Stir in chopped garlic chives. Add 1/4 tsp. ground chiles and 1 Tbsp. peanuts and toss well. Transfer to serving plates.

Garnish with remaining 1/4 tsp. ground chiles, 1 Tbsp. peanuts, and lime wedges.