To prepare pork: Combine 5 cloves grated garlic, 2 tablespoons brown sugar, chili flakes, 1 tablespoon kosher salt and pepper. Rub marinade all over pork. If you have time, cover and refrigerate for 1 hour to up to 24 hours. Otherwise, proceed with recipe.
Set electric pressure cooker or slow cooker to sauté (or use a large skillet). Add pork in batches and sear until browned all over, about 2 minutes per side. Add 3/4 cup water to pot (or to skillet to deglaze, then move to pot), cover, and set to cook for 90 minutes on high pressure or 5 to 7 hours on high in a slow cooker.
To prepare sauce: In a small pot, warm peanut oil over medium heat. Add 4 cloves grated garlic and ginger, and sauté until fragrant, 1 to 2 minutes. Add gochujang, soy sauce, ketchup, mirin, honey, 1 tablespoon vinegar, fish sauce and 1 teaspoon sesame oil. Bring to a simmer. Cook until thickened, 1 to 2 minutes. Set sauce aside. (It can be made up to a week ahead and stored in the refrigerator.)
To finish pork: If using a pressure cooker for pork, manually release steam. Let pork cool until you can handle it, then shred it into bite-size pieces. Pork can be made to this point up to 3 days ahead.
While pork cools, strain liquid from bottom of pot. Pour off fat (or chill liquid, then scoop off solidified fat with a spoon). Reserve.
To prepare cucumbers: In a small bowl, combine cucumbers, 1 1/2 tablespoons vinegar, 2 teaspoons sesame oil, 2 teaspoons brown sugar, sea salt and onion. Let sit, tossing once or twice, for at least 20 minutes. Stir in sesame seeds.