Preheat oven to 375°F; grease a 9x13-inch dish. Slightly overlap two crusts on lightly floured surface; roll together into rectangle to line dish bottom and sides, then fit in dish.
Whisk soup, milk, broth, and flour in large saucepan over medium about 5 minutes until smooth and thickened.
Stir in chicken, frozen vegetables, thyme, garlic powder, pepper, and salt until combined.
Pour over bottom crust.
Overlap and roll remaining two crusts into rectangle to cover dish; place over filling. Crimp edges to seal; cut several vent slits.
Bake 40-45 minutes until crust is golden brown and filling bubbly.
Cool 10 minutes before serving.