Place the rice noodles in a large bowl and cover with warm water. Soak until flexible but still slightly firm, about 20 to 30 minutes, then drain. Follow the package directions if they call for a different method.
Whisk the tamarind paste, fish sauce, brown sugar, and optional chili flakes with 2 tablespoons of water until the sugar dissolves. Taste; the sauce should be tangy, salty, and lightly sweet.
Pat the shrimp dry. Beat the eggs in a small bowl. Set the bean sprouts, scallions, peanuts, and lime beside the stove so they are ready to add quickly.
Heat 1 tablespoon of oil in a large wok or skillet over medium-high heat. Add the shrimp in one layer and cook, turning once, until opaque and cooked through, about 2 minutes per side. Transfer to a plate.
Add the remaining oil and garlic. Stir for 20 seconds, then pour in the eggs. Scramble just until softly set. Add the drained noodles and sauce; toss continuously until the noodles are tender and glossy, 2 to 4 minutes. Add a splash of water if the pan looks dry.
Return the shrimp to the pan. Add all the bean sprouts and half the scallions; stir-fry over high heat for 2 to 3 minutes, until the sprouts are fully heated through and steaming hot.
Divide among four plates. Top with the remaining scallions and chopped peanuts. Serve with lime wedges; diners can adjust the sweet, sour, and chili balance at the table.
