As a suggestion for a general re-design to solve this issue... Cluster the consumable components together. Flash and battery. Attach the storage via iSCSI or

As a suggestion for a general re-design to solve this issue... Cluster the consumable components together. Flash and battery. Attach the storage via iSCSI or something similar, have the car's main computer perform encryption/decryption (rather than trusting a component expected to be swapped). In the case of a battery swap the bulk contents of the (encrypted) external storage can also be cloned. That can probably happen in the timespan that a battery is topped up to charge, but would be more difficult for automated swaps at refueling points. A home/cloud backup and transferring only the new data might be sufficient.