I hear that. I ditched my panniers for a backpack a number of years ago; I use a Marmot Ultra Kompressor which I would suspect would hold nearly everything you have listed except for the laptop. I like that the backpack keeps the weight up high which doesn't affect handling, and that I have nothing to detach from the bike when I get to work. I live in the Midwest and also experience triple digit heat and high relative humidity during the summer...sweaty back does suck but a synthetic top - I wear a tri singlet - helps this.
Either way, best of luck on the new commute, hope all goes well.