Eh, except this general pattern has been mostly worked through for ages in other devices.
Don't over-write the existing firmware, have a separate partition, the user transfers the update from phone/tablet/computer over BT or a cable. System powers up, confirms valid and complete download package, and only then initiates the update. Backup of original flash can be done on-system or to user device. Flash the update, if system check fails, revert. There are always edge cases (e.g. bricked Xbox, PS, ...), but it's a general pattern many use without issues. Why should someone be able to update their phone and tablet (and PC/Mac) operating systems AND apps, but be unable to do a much simpler bike firmware update?