If only we had that choice. It's summer or never, so we'll make the best of it. We're used to extreme summer humidity. Where we live it's 80-90% every day in summer.
We find we do best if we acclimate ourselves as much as possible. Our first summer heat trip we did everything wrong; we approached it the same as our spring visits. (= afternoons in the parks)
We weren't going to attempt it a second time, but it was kind of our only option one year.
It happened to follow a visit to the beach, and THAT made all the difference. At the beach, we were all out in afternoon heat, and no AC where we stayed.
For us, we've also discovered in summer it pays to eat lunch and dinner late.
I'm no fan of FP+, but it means there's an even bigger evening crowd drop off than before. At night, we walk on many rides. Most WDW visitors try to stick it out in the afternoon, or they bail when the storms hit. Post storms, the parks are empty and lovely. Pre-storm the high heat and humidity, plus hour waits for everything make (many) folks miserable.
Now some days, the storms hit early, or they last 2pm- past 9pm. Even same day weather predictions are often 'off', but anything more than 24 hours out is just a guide. (=another downside of FP+) Many rides stop operating during thunder= no FP. Many storms last 10 min, others are torrential for hours.
Knowing all that in advance = smarter summer touring. DO bring water tolerant shoes to WDW, and plastic ponchos. Some type to Teva-like sandal is great. Sneakers are often horrible. They may take days to dry- and smell horrible! (lesson learned!) Every summer trip we get in a ride/show next to a family with putrid shoes! Ick! Some days it also pays to wear thin easy dry nylon shorts...if no rain...guys cargo shorts are awesome. Big pockets = awesome!
Heavy backpacks are also miserable. Less= more. Your back and everything sweats.
Water is essential! Drink! But keep the outside of your body DRY. Misters make you look and feel sweaty. Wicking fabric is great. Silk (and similar) is great.