Return air balancing

I have a 3 level townhouse, there is a damper controlling flow to each floor. My main concern is the number of vents on my upper level. One master bedroom (with bath), 2 smaller bedrooms, and another bath. The return air is in the hall. There is one vent in each smaller bed room and one in the hall bath. Then there are 2 in master bath, 1 in walk in closet in master, 4 smaller vents in master, and 1 giant vent in the master. As you can imagine with 8+ vents in the master and only 3 in the other upper areas, most of the air goes to the master. It is freezing in the summer and toasty in the Winter. The air pressure keeps the master door closed if it isn’t held open.

I can easily see where all the vent lines are coming from from the attic. What I’ve been wondering is if the large vent in the master is supposed to have been connected to the return. The vent it self looks like it should be a return grille and it is positioned closest to the door.

Do my suspicions sound correct? Has it been installed wrong? Could I pinch it off to see the difference without much issue to my air handler? Would it be best to have it attached as it’s own return air for the master bedroom?