My wife shot weddings for over 20 years along with events and location work, in addition to her usual studio work. I helped on her big shoots while adding industrial, aerial and scientific shooting to our business. Very long days with mobility a premium and usually at least two cameras ready at any moment.
For us the solution was long camera straps worn across our bodies and adjusted to ride on each side slightly above belt level. That allows us to stop camera swing with the press of an elbow while moving, yet keeps the cameras quickly accessible, even as the one not in use is down and out of the way. Wide strap padding is mandatory. We're heavy users of belt pouches for immediate access to lenses and accessories, further limiting weight on shoulders.
In our experience camera bags and slings are especially hard on shoulders and spines, while backpacks aren't much better. Worse yet, if you have to root in a sling, bag or pack for something, you usually miss the shot. We keep our full array of gear for a shoot in roller bags, stow those in a safe central location, and return to them to change gear between sessions. Handy, but not on our backs. We usually carry a small day pack, but it's reserved for food, water, rain gear, etc. The transition to landscape shoots is pretty direct.
Everybody has to work out their own system, but ours revolves around keeping the weight off our necks and spines but accessible and minimally floppy while we're moving around. Even with all those spine/neck precautions, the central feature of our business is an insurance policy that covers good chiropractics.