Add the macaroni, water, salt into the Instant Pot insert.
Top with pumpkin purée, butter, onion powder, and pumpkin pie spice but do not mix
Place lid on the Instant Pot, turn the valve to sealing, and select pressure cook (or manual) high pressure, for 4 minutes.
Allow for a Natural Pressure Release (NPR) of 5 minutes and then do a Quick Pressure Release (cover the vent as it will spurt everywhere).
Open the lid and quickly stir in the cheeses, warm milk, and cream. mix until combined and cheese is melted, then adjust seasonings, add any toppings or mix-ins, and serve.
Notes
Do not mix the pumpkin in before cooking or you will end up with the dreaded burn notice.
For extra flavor, you can use reduced-salt chicken broth or vegetable broth instead of water. Just keep an eye on the amount of salt you add in then.
Keep the time under pressure to 4 minutes, any longer than that you will probably end up with mushy pasta.
Use store-bought shredded or grated cheese to save some time on this meal.
You don't need to add two different kinds of cheese; it's fine to use one if that is what you have. Or change out the types of cheese to suit your taste.
Cooking time does not include the time it takes for the Instant Pot to come up to pressure or for the pressure to release.