75F is too cold for ball pythons. Also, if you put in a lamp chances are you will zap humidity. Poster board does not insulate well

I would get cork board or the pink rigid insulation and insulate the tank on the three sides and see how much this helps the situation.

If more heat is needed, perhaps a CHE would work as they don't zap as much humidity, especially when you seal most of the top of your tank.

Also, make certain that your substrate is not too thick and that the UTH is the appropriate size for your tank

Lastly, why shut off the heat????