Turn on the Instant Pot’s saute function to Normal / Medium.
Add chopped bacon and saute until crisp, 8 to 10 minutes. Remove bacon with a slotted spoon and transfer to a paper towel-lined plate.
In remaining bacon fat in the Instant Pot, saute the onions, carrots, celery, chili powder, garlic powder, thyme, and salt for 4 minutes to get them started cooking.
Turn off the saute function.
Slowly pour stock into the Instant Pot, scraping up any browned bits on the bottom of the pan. Stir in the potatoes and corn.
Close and lock the lid.
Set the pressure valve to Sealing.
Cook on manual / high pressure for 5 minutes.
Manually release any remaining pressure.
Use an immersion blender to blend about half of the soup right in the Instant Pot. The more you blend, the creamier and thicker the soup base will be. (Alternatively you can remove 2 cups of the stock (including potatoes and corn) and blend it in a standing blender until smooth. Stir the blended portion back into the soup.)
Stir the milk and chicken into the chowder and let it sit for a minute or two to warm up.
Taste chowder and season with salt and pepper, if needed.
Serve chowder with cheese, sour cream, chives, and bacon on top.