Ok, for what it’s worth, we have something very similar to this Eddie Bauer cross-body bag, and for shorter hikes like you describe where less stuff is required, it will easily hold water, snacks, wallet/keys/iphone and full-sized birding binoculars (you said you carry a camera, but that can be quite a size-range). Adding a layer (shell, pull-over) gets tricky because if you can’t cram it into the bag itself - which may not be possible depending on the size of the jacket and how big your ditty is - there’s no easy way to secure the shell/jacket to the pack.
I also have and use this:
https://www.rei.com/product/184952/osprey-talon-6-waist-pack-mens which holds the same amount of stuff, but it’s much easier to strap a soft shell or other light jacket to the outside. It would probably be a bit wonky as a cross-body bag, but it’s also far more comfortable FOR ME if I’m doing something that requires using poles, or using my hands to climb.