Ok, thank you for the pointer, so I’d need to know what flash-kernel does, in order to be able to recover if something goes wrong.
I found Librem5 / flash-kernel · GitLab but I’m sorry the readme only added to the confusion. It is missing in its overview (some words of) what kind of actions it performs and for what reason.
It only says it flashes the kernel and init ramdisk to some untold place, but later also talks about some quite extensive amounts of uboot script magic that it supports. For example, .deb package hooks and /etc/flash-kernel override.d directories.
I first assumed it would flash the files from /boot into yet another location (where?), but this forum thread suggested some leftovers in /boot there were actually created by flash-kernel: Old kernel files in /boot
Be ready to boot your device over USB with uuu in case something goes wrong.
I think booting the jumpdrive system in order to telnet into that is easy enough, but would that rescue system actually allow to run flash-kernel to fix things? Because the “flashing” part of this tool makes me think it’s not enough to simply rename boot.scr (directly on the /boot partition) to get a default rescue boot, or restore a backup copy of it, or is it?