Cut diagonal lines across the pork belly skin in 2 opposing directions, forming a diamond pattern.
Cook and stir water, salt, sugar, soy sauce, garlic, and ginger in a pot over medium heat until salt is dissolved, about 5 minutes. Let marinade cool, 20 to 30 minutes. Pour into a resealable plastic bag. Add the pork belly to the marinade and refrigerate for 24 hours.
Drain pork belly, discarding marinade; rinse pork belly, pat dry, and place into a slow cooker. Coat with barbeque sauce and cook on Low until meat begins to fall apart, about 8 hours. An instant-read thermometer inserted into the center should read 145 degrees F (63 degrees C).
Cook and stir the pork belly in a skillet over medium-high heat until crispy, about 5 minutes. Transfer to a cutting board and shred with forks. Place shredded pork belly on tortillas and top with red bell peppers, radishes, pickled onions, cilantro, and sriracha sauce.