I think there must be a reason why the disk switch has been protected with a firmware password.
To do e.g. a startup disk modification, you still need an admin password, because you need to call the command with a sudoat the beginning.
e.g. sudo bless --mount /Volumes/Startup_Disk_Name --setBoot
And if you can get this command to work without an admin password, you'll be asked for the firmware password instead, when starting up on the other disk.
--setBoot Set the system to boot off the specified partition. This is implemented in a platform-specific manner. On Open Firmware-based systems, the boot-device variable is modified. On EFI-based systems, the efi-boot-device variable is changed. This is not supported on Apple Silicon based systems.