Is the cable actually directy buried and not in a pipe? It should be in conduit wherever its underground or exposed to potential damage like Your weed wacker. It looks like that pipe goes underground which is why i wasnt worried haha.
That steel conduit is just there for mechanical protection. The cable inside the pipe has its own insulation and you can assume its integrity is still good if its still working fine. I wouldn’t upgrade the protective device myself. If youd had no issues with the circuit then all id do is get a galvanised cable/pipe protector and cover the rusting pipe. (Im an electrician 🤙)