We did WDW in July last year and August the year before. We're doing June this year - maybe it will be a touch better, but I'm not planning on it.
I learned a lot one year to the next. My biggest recommendation if you're doing a park day is this: have a reservation for lunch on park days. Hit the parks in the mornings - the earlier the better. By lunchtime, the heat will be getting unbearable and everyone will be hungry and tired. Having a place to go in the a/c to eat and take a break really refreshed everyone. We would head back to the hotel after that and then maybe go back to the park in the evenings after the sun sets.
If you don't want to do table service and prefer QS to save money, plan to eat lunch at 11 am sharp, when most of the lunch QS restaurants open. If you wait until noon, there likely won't be any tables inside in the a/c, and you'll all be hungry, hot, and stuck with eating chicken strips and French fries in the blazing hot sun . . . that's a recipe for meltdowns.