Regarding backpacks, I dislike 'em but that's mostly because of chronic back and neck pain from an old injury. Decades ago I used backpacks to carry a few items, like jeans and a casual shirt to change into at work.

Nowadays I use a cargo net to stash whatever I'm carrying on the rear rack. A few cycling friends use rack packs that unhook easily.

I'm in Texas where summers are about as hot as it gets in the U.S. Many of my cycling friends prefer backpacks even in summer on casual rides. Yup, they get hot and sweaty. I suppose they'd carry at least a change of shirt for commuting.