Make the ponzu Thousand Island: Whisk all dressing ingredients together until smooth. Chill until ready to use (it thickens slightly as it sits).
Season pork lightly with salt and pepper. In a bowl, whisk ponzu, guava marmalade, miso, chili paste, garlic, oregano, cumin, lime juice, orange juice, and broth. Add to the Instant Pot with the pork. Seal and cook on High Pressure for 70 minutes. Let pressure release naturally for 10 minutes, then quick release.
Remove the pork to a bowl and shred. Add some of the cooking liquid and toss to coat.
Lightly toast gluten-free buns. Spread a generous layer of ponzu Thousand Island on both sides. Pile on the pulled pork, then top with cabbage and pickled onions.