The process for the phone is modeled after the process for the dev kit.
Booting over USB or microSD will not be supported by default (while it is a Linux phone, the lower levels are not very similar to x86 PCs yet), which gets rid of most accidental bricking issues and any potential security problems.
Flashing the image will be done by attaching the Librem5 to another computer, and running the flashing program with a downloaded OS image there. This will be doable even with a totally wiped L5.
The partition layout is pretty simple on the current images: MBR partition table, a boot loader at a magic location, a /boot partition, and a root partition. There’s been some work to get the boot loader into an area more difficult to brick and allow GPT, but it stalled.
PS. Booting from USB/SD will not be impossible, but it will need some boot loader changes which we so far aren’t planning to have.