We just got back and I was WRONG

I'm so wrong that I'm considering booking for the same post-july 4th week again for next year haha
Sounds silly but I did not realize that low crowds didn't just mean shorter wait times in the park. There were so many times during the trip where I said, "where is everyone???" I walked right up to the check-in desk, there were minimal lines at the food court and plenty of open tables, and the buses weren't crowded either. In April, check-in took ~30min, the food court was busy although manageable, and the bus was standing room only even though we rode at off-peak times. I was pleasantly surprised that I was able to book sci-fi dine in and oga's cantina on a whim and I didn't just get lucky - there was plenty of availability throughout the day.
Disclaimer - our July trip was a resort-hopping trip with one park day and LL and our April trip had 3 park days and no LL so not exactly apples to apples comparison. The heat was very manageable in the park - I prioritized outdoor attractions for the morning and did shows in the afternoon and walked from air conditioned space to air conditioned space. Resort-hopping felt hotter because there were fewer AC breaks and my kid wanted to spend most of the time digging in the sand/playing lawn games/on the playground.
I originally thought the heat would be miserable because we live somewhere with similar humidity to Orlando but just a few degrees cooler, and most of our outdoor time has no quick access to AC (ex. after leaving the playground and getting into the car, you still have to wait for the AC to kick in vs at Disney, you merely walk into the hotel lobby and are greeted with an instant blast of cold air). It sounds really small but these quick AC breaks make a huge difference. At home, I constantly worry about my kid overheating but I was surprised to find that it wasn't the case while we were on property.
As others noted above, I think it REALLY depends on your heat tolerance. If you can walk 10-20min outdoors in 90 degrees then you'll be fine. Wear the right clothes, hydrate, and have a good heat plan and the lower crowds make for a more enjoyable trip.