I know a few people who can ride for hours in blazing heat. I don't know how they do that. Must be genetic or many hours of intensional training to allow the body to do that. Until I can get out of this hellhole, I think the only answer is to keep my rides under 90 minutes.
I wear a 3L pack and two 24 ounce water bottles on these hot days, one of the bottles is filled with one of the many electrolyte mixes I have tried. I "camel up" the day before by drinking as much water as I can stand, combined with supplements. after 90 minutes, my water is totally depleted and I start to get a headache. this has been going on for a decade with no relief. I asked my doctor and he said "drink some Gatorade."