Wash and dry all produce. Trim and thinly slice radishes and cucumber into rounds. Trim and thinly slice scallions. Pick cilantro leaves from stems; discard stems.
In a medium bowl, combine radishes, cucumber, vinegar, 1 tsp sugar (2 tsp for 4 servings), and a pinch of salt and pepper. Taste and add a pinch more sugar if you like a sweeter pickle. Set aside, stirring occasionally, until ready to serve.
In a small bowl, combine sour cream and a pinch of chili flakes (taste and add more from there if desired; save a little for steps 4 and 6). Season with salt and pepper.
Heat a drizzle of oil in a large pan over medium-high heat. Add scallions and cabbage; season with salt and pepper. Cook, stirring, until softened, 4-5 minutes. Add beef, garlic powder, and 1 tsp sugar (2 tsp for 4 servings). Cook, breaking up meat into pieces, until browned and cooked through, 4-5 minutes. Stir in sesame oil, half the soy sauce (use all for 4), half the sriracha, and a pinch of chili flakes to taste. Taste and season with salt, pepper, and up to ½ tsp more sugar.
While filling cooks, wrap tortillas in damp paper towels and microwave until warm and pliable, 30 seconds.
Spread a bit of chili sour cream onto each tortilla, then top with filling, a bit of pickled veggies, cilantro, and as much remaining sriracha and chili flakes as you like. Serve with any remaining pickled veggies on the side.