More important to get the return air from the hot space. The house that sold I had made some changes. Originally the 2nd floor return register was near the floor. I added a second register up high near the ceiling. In the summer I would open the high register and close the low register. I would set the fan control on the thermostat to on rather than auto. I also assed an inline fan on the return air duct from 2nd floor. And I would block the 1st floor return register by about 30%. I also throttled back the register near the stat Before doing this the 2nd floor would be 10 degrees hotter than 1st flood. Not good for sleeping at night. After I made the changes 2nd floor was about 2 degrees cooler than 1st floor.
But I did check to besure there was proper air flow across the A coil of the AC.
Now my new house I need to keep everything open and not add any resistance. My ducts are under sized. 15 inch return and supply going to the A coil. But for the size of the unit the ducts should be 20 inch.
You need a remote thermostat.
One made a HUGE difference in the comfort level of our house.