Without automatic A/C there is really only volume of air control. The temp controll will only affect the cycling rate. It will increase duct temp but it will still be A/C. Modern systems blend heat and A/C to get warmer temps.
You will have to find a way to control the mix with a combination of duct control and heat. Closing the center duct should reduce the temp. If you add some heat, it should go to the outer vents. I am not familiar with your controls so I am guessing.
Bosch Master, ASE Master, L1
33 years MB technician