I've done several 100-200 mile gravel rides in the midwest in the 80s and 90s with high humidity and a high dewpoint.
After several years I've dialed in my nutrition and hydration but I still haven't found the perfect formula. But here are some things that I have learned. They may or may not work for you.
First -- acclimatization is huge. A few weeks before my biggest ride I'll start doing some sauna training.
Second -- drinking pure water for several hours without replacing electrolytes and nutrients is a good way to go to the hospital. (See hypernatremia.) I've had three friends go to the hospital either during or directly after Dirty Kanza. I drink a drink mix now (Skratch), but fairly well diluted because sugar gets old after several hours in the heat.
Third -- sugar gets old after several hours in the heat. Have a variety of nutrition options. Savory/salty options (sweet potatoes and peanut butter, for instance) work for me. A well-timed bag of potato chips works wonders.
Fourth -- I never know exactly what my body is going to do after 100 miles and with the heat all bets are off. So I have a variety of options. I try to stick with foods that I've tried in the past and I know that my stomach will tolerate, but I usually throw one or two different things in there because who knows what's going to sound good out there? This year at Dirty Kanza I had lots of medjool dates, some Epic bars, a couple peanut butter/honey ball things, a couple mini-pies from the Feedzone Portables cookbook, and these little Rice/Ginger ball things from the Feedzone Portables cookbook. As it turned out, the Rice balls were the only thing I could choke down 8 hours into the ride, in the worst of the afternoon heat. The ginger helped calm my stomach down, apparently.
Fifth -- Your body can only digest about 250 calories/hour (plus or minus 50) and you will burn more than that, so don't try to replace calories. Just fuel. Rule of thumb I shoot for is 200-ish calories of mostly carbs every hour, with some protein every third hour.
Sixth -- this is just my stomach but I can't deal with dairy before, during, or after a ride. But especially during.