DD and I were in WDW one August for 10 days. It was so hot and humid, heat index was into the hundreds every day. We are not morning people, so rope drop wasn't going to be fun for us. We tried to head OUT of our hotel between 10:30am and noon every day. One of the first things we'd do in the park is have lunch. It's amazing how many people leave after lunch time. Sure, it put us there in the heat of the day, but wait times were short. Take as much advantage of attractions with a/c as you can! We tried to do a lot of the "inside" things, too, alternating them with attractions that involved waiting in line outdoors. IF you feel the need for a break, sit indoors and have a cold drink (yes, it can be done even in the time of online ordering- just put away your phones and tell them you can't order online- which I can't, no phone- and they let you in). DD and I would get something small to share (fries) and then each get a big, cold drink and sit in the a/c until refreshed. We also found that we needed a dinner break around 7:30pm, someplace where we could sit down in the a/c, relax, and refresh for the evenings in the parks- because it's not that much cooler at night, there is just no sun! We found that we were there enough days to take our park days at a much slower pace, but we did everything we always do!
Heat tip: Every morning I'd spray my feet with antiperspirant. They didn't sweat as much, and dry feet are feet that are less likely to blister. I also used stick antiperspirant on anyplace that might tend to rub or chafe- or collect sweat (a swipe down my spine and under my bra-line, for example) with a dusting of baby powder after. Some people use something called BodyGlide (I think). Anything to cut down on sweating.
We also used frozen washcloths. Fold washcloths in quarters, wet, and put in individual sandwich bags in the freezer at bedtime. I had an insulated lunch bag (big enough for a couple of sandwiches) that I'd put the frozen cloths in to take into the parks and they'd stay cold/cool for quite a few hours. It also helped just to have the cloths to wet in a sink and then wipe over our forearms, necks, etc. Reasonably cooling.
I am not a sunglasses/hat girl, but both are helpful with the glare from the summer sun. Also bring sunscreen and use it regularly. That stuff will sweat off- and you are gonna sweat!