Be careful on the sunny room. It will usually be warmer than the rest unless zoning is implemented. Cutting airflow in winter to cool the room off would make it hotter in summer. Our master bedroom we like it cooler year round. A simple manual damper in the attic is what I used. Open fully by summer, open about 1/2 way in winter. "redneck zone control" if you will
Location of the return helps, but it's normally determined when the house it built. Low returns help in rooms that are cold all the time, high returns help in rooms that are hot all the time. I considered installing a high return to our master bedroom, but the damper valve was easier to install.