I've gone back and forth on backpacks. Currently I am in the pro-backpack camp. Lately I've been using a small Camelbak plus a bottle for my rides, and just got a bigger Camelbak (particularly so I don't need to stop anywhere on a longer ride during the pandemic). I switched from commuting with a pannier to commuting with a backpack.

If you feel hotter wearing something on your back, well, I can't say your feeling is wrong, but the temperature gets north of 105°F out on the road here, and I don't think the Camelbak makes that any worse. I've noticed I do drink more frequently with a Camelbak.