I am sorry, but Unified Bootloader is the only bootloader available for the S32K3 family and is an unofficial demo only shared by individual on the community. It is provided AS IS without any guarantees and no support. It is not production code.
I am afraid there is no specific information about the S32K358 bootloader. The only one available is the Unified Bootloader, examples of other derivatives can be created based on this existing example.