Hello there,
I am sorry that you haven't gotten a response so far, and I believe there must be solutions to make your situation cooler. Binders and Sports bras are generally made of spandex which usually makes them hotter in humid and hot temperatures. I wonder if it is an helpful option to wear a binder made of relative natural material like cotton?, from my past experience the cotton ones are generally more breathable and comfortable. I also live in a place where the temperature goes up to 100 degree in summer (Sacramento), and quarantine actually helped a little with the heat problem, becuase I don't have to go out as often. In the cases of reducing boob size, I have tried running and jumping robes to lose weight and the boobs are usually the first place where I start to lose weight.
I reaserched and found some links that might be helpful and practical:
How to survive chest binding in the summer: 30 must try tips
https://spectrumoutfitters.co.uk/blo...-while-binding
The 6 best tips for binding in the summer
http://point5cc.com/the-6-best-tips-...in-the-summer/
I hope you enjoy summer and the coming fall.