Stay with the pelicase. The fact that it's getting beat up is telling you something.
There are a bunch of different sizes. I use one that is roughly 12" x 10" x 4" for my canoe expeditions. I've done caving too, and that has to be the most camera hostile evironment short of underwater. Pelicases also can be bought in high visibility colours. Having a black or grey case in a cave can be a problem. If you do have a dark case, get some highly reflective stickers to put on it. Will save it getting kicked, may help you find it if you drop it.
However: Chuck the foam lining. If it ever gets wet or dirty it is impossible to dry or clean.
For my canoe use, I bought a small roll of that reflective heavy duty bubble wrap that DIY stores sell for insulating your water tank. Aluminum coated about 1/4" thick.
Using this, I lined my case, using double sided carpet tape to hold it in. Edges were sealed with duct tape. Secondary layers shaped into U's were taped in place to partition. spare data cards, lens cleaner etc, were dotted with velcro, and placed where they wouldn't interfere with the camera.
One other piece of advice: Try to use constant length lenses. These won't change volume, pulling soggy wet cave air into the lens as you focus or zoom.
Find the smallest case that will work, and then put THAT in a backpack. That will make carrying easier for the long dull parts of the cave, but still protect the camera.
For a cave environment,I'd aslo suggest adding desicant packages (Lee Valley Tools) to try to keep the interior of the case below saturated humidity.
You also want hand cleaning materials outside of the case (shampoo bottle of soapy water and a small towel) to clean your hands and your face before using your camera. Mud never sleeps.
