Add the parboiled rice, vegetable broth, fire roasted diced tomatoes, tomato sauce, okra, celery, carrots, scallions, onions, garlic and all seasonings to the slow cooker. Stir gently, ensuring the rice remains covered by the broth.
Set to LOW for 3 hours. When done, mix well and turn the heat to HIGH. Add the shrimp, cover and cook for additional 30 minutes.
Transfer to a serving dish and serve warm, seasoned with additional salt, black pepper and cayenne powder if desired.