Pat chicken breasts dry with paper towels. Season both sides generously with garlic powder, onion powder, smoked paprika, salt, and pepper. If breasts are thick, pound to even 3/4-inch thickness.
Heat olive oil in a large skillet over medium-high heat until shimmering. Add chicken breasts and cook undisturbed for 5 minutes until golden brown. Flip and cook 4 more minutes. Transfer to a plate and set aside.
While chicken cooks, rinse rice under cold water until water runs clear. In a saucepan, bring chicken broth and salt to a boil. Add rice, reduce to lowest heat, cover tightly, and cook 18 minutes without lifting lid. Remove from heat and let steam 5 minutes, then fluff with a fork.
In the same skillet used for chicken, melt butter over medium heat. Sprinkle in flour and whisk constantly for 90 seconds until light golden and fragrant.
Gradually pour in milk while whisking continuously, followed by chicken stock. Bring to a gentle simmer, whisking frequently, until sauce thickens (about 3 minutes). Stir in garlic powder and thyme.
Reduce heat to low. Add cheddar and Parmesan cheese gradually, stirring until completely melted and smooth. Taste and adjust seasoning as needed.
Return seared chicken breasts to the skillet, nestling them into the sauce. Spoon sauce over the tops, cover pan, and simmer on low heat for 12-15 minutes until chicken reaches 165°F internal temperature. Shake pan gently occasionally to prevent sticking.
Let chicken rest in the sauce for 3 minutes. Spread rice on a serving platter, arrange chicken on top, and ladle sauce over everything. Garnish with fresh parsley and serve immediately.