If mine does it, I'm thinking of using some FRP panels as the backer to replace the cardboard.
FRP comes in different thickness/stiffness and is 100% waterproof. I'm thinking the thinner panels will handle the bend needed and I would build a wooden jig to hold that bend shape and then epoxy the stock fiberglass panel to the FRP clamped on the jig. Once set, it should hold the curve. FRP comes in 4x8' panels, but with it bonded to the front panel, that joint will not matter as its in the flat portion of the panel. Should be a drop in repair after that. Basically the same thing your repair shop did, but using FRP instead of thin plywood for the base (100% waterproof that way).
I've watched a couple of youtube videos where front caps on multiple brands were replaced with everything from FRP to aluminum roll panels. Assuming the framing underneath is sound, the biggest hassle is finding somewhere to do the work to keep it dry in case of rain as it may take more than one day. It will take 24 hours for epoxy to cure and the front cap will have to be off to epoxy it so it's at least a 2 day project (probably more since its not like I've done this before).
Just the 2 of us in a...
2015 Freedom Express 246RKS
2002 GMC Sierra 1500