We vacationed in WDW every August while my kids were in grade school. It’s very hot & humid and mid day breaks are a must do. Buy a case of water and insulated carriers for park visits. Drinking only water will make a difference in your tolerance to heat.
Since our vacation schedule no longer requires an August vacation, we stay away from summer heat and go early and late in the year. If you enjoy Christmas decorations, I’d stick with November for the nice weather and the Holiday atmosphere.