Cook spaghetti in salted boiling water until al dente, reserve 1/2 cup pasta water, drain.
Pat chicken dry and season with salt, pepper, garlic powder, smoked paprika, and Italian seasoning.
Melt 3 tbsp butter in a large deep pan over medium-high heat.
Sear chicken strips 3-4 minutes per side until thick-cut pieces are golden-brown with crispy mozzarella-Parmesan edges. Sprinkle 1/4 cup Parmesan on top of chicken in the last minute of cooking. Remove.
Add remaining 2 tbsp butter and garlic, stirring 30 seconds.
Pour in heavy cream and bring to a gentle simmer.
Whisk in cream cheese until fully melted and smooth.
Add mozzarella and remaining Parmesan one handful at a time until the sauce is visibly thick, glossy, and velvety. Loosen with pasta water as needed.
Add spaghetti and toss until every strand is drenched in mozzarella Parmesan cream sauce.
Plate chicken on one side and spaghetti on the other, finish with parsley.