One of my least comfortable visits to WDW was in May. that was about 15 years ago. Since then I've had many happy summer visits.
My biggest suggestion is to try to acclimate oneself as much as possible before you go. Within reason, the more you can get outdoors on sunny, warm days the better. Within reason, when indoors, try to limit AC, at least if you are used to using AC at home in May.
It takes about 2 weeks to get used to warmer temps, but just going outdoors for lunch is something.
When I visit in the summer, I do take afternoon breaks: leaving by 3pm at the latest, and returning around 5:30pm at the earliest. It often depends what time the afternoon storm hits. A good rain cools things down, but it is also nice to be at the hotel when it hits. I don't mind getting wet, but water-logged shoes can be a pain. As others have said, swimming is one way to cool off.
The heat tends to make people a bit drowsy, especially after eating and/or drinking alcohol. In the summer, it is good to avoid too low-blood sugar, while being careful about high glycemic foods like french fries.
Fresh fruit/produce is good. Another great snack- of sorts -is chia seeds. There used to be a product called Chia shots. Those were perfect for WDW, but they are no more.
Conversely, I've sometimes found limited sweets help if anyone feels a little woozy from the heat. Apple juice and lemonade can be lifesavers. Of course, it pays to drink plenty of water, but I now try to buy electrolyte water and apple juice or other fruit juice. Sometimes WDW has pineapple juice.