In small bowl, combine 1 teaspoon ground cumin, 1 teaspoon garlic powder, 1 teaspoon chili powder, 1 teaspoon paprika, and ¼ teaspoon kosher salt. Rub spice mixture all over 1 pound boneless skinless chicken breasts until fully coated.
Set Instant Pot to Sauté mode. Add 2 tablespoons olive oil. Once hot, add ½ white onion, diced, with pinch of salt. Sauté 2–3 minutes until softened. Press Cancel.
Add seasoned chicken breasts to pot. Add 1 four-ounce can green chilis, 1 cup red enchilada sauce, 1 teaspoon ground cumin, 1 teaspoon garlic powder, ½ teaspoon kosher salt, and 6 cups chicken broth. Do not add masa harina.
Secure lid; set valve to Sealing. Cook on High Pressure for 12 minutes. Perform Quick Release when cycle finishes.
Remove chicken breasts and shred with chicken shredder or two forks. Set aside.
Ladle approximately 2 cups hot broth into medium bowl. Whisk in ½ cup masa harina until completely smooth. Pour slurry back into pot and stir well.
Set pot to Sauté mode. Add shredded chicken. Simmer 5–7 minutes, stirring frequently, until soup reaches desired thickness. Press Cancel.
Allow soup to drop slightly from active boil. Stir in 4 ounces cream cheese, softened and cubed, a few pieces at a time, until fully melted. Stir in 2 cups shredded cheddar cheese until melted and smooth. Taste; adjust salt as needed.
Ladle into bowls. Top with extra shredded cheddar, tortilla strips, cilantro, and sour cream as desired.