Cut roast into 4 quarters and season liberally with salt and pepper. Place a large cast iron pan over high heat and coat with olive oil; heat until smoking. Place meat in the hot pan and turn to sear on all sides until a crust forms, 5 to 10 minutes. Transfer seared meat to a slow cooker.
Add bitter orange marinade, garlic marinade, onion, garlic, and cumin to the slow cooker. Make sure meat is mostly covered in marinade.
Cook on Low for 6 hours; shred meat in the slow cooker using tongs or a fork. Continue cooking on Low for another 1 to 1 1/2 hours. Add additional salt or cumin if desired.