Heat the olive oil in the inner pot at on Sauté. Add the sausage slices, and cook until browned, about 2 minutes per side. Transfer the meat to a paper towel–lined plate to drain.
Add the onion, bell pepper, celery, and garlic and cook, stirring occasionally, until the onion is translucent, about 3 minutes.
Add the Cajun seasoning, thyme, and rice, and stir until the rice is well coated in the oil. Press Cancel to stop the cooking.
Add the tomatoes and their juices, the stock, and salt. Stir with a spatula freeing any food particles sticking to the bottom of the Instant Pot insert.
Lock the lid. Cook on Manual High Pressure for 8 minutes, then allow a Natural Pressure Release for 5 minutes followed by manual release of the remaining pressure.
Stir the rice mixture, then stir in the shrimp, scallions, and reserved sausage. Replace the Instant Pot lid and allow the ingredients to heat through for 5 minutes in the residual heat.