Cool. You probably should mark either my post or your post as “solution”, in case someone else wonders the same thing. (Really it is only the two posts together that provide the full information needed to answer the original question.)
While we are on the subject of logging, I note that out-of-the-box the Librem 5 is not ever cleaning up the journal, which for me meant that tracking anything down in the journal was tediously slow. So I changed the configuration to do at least some cleaning.
In case it helps anyone else: sudo journalctl --disk-usage
will show that the journal is getting BIG. Edit /etc/systemd/journald.conf and set SystemMaxUse and/or MaxFileSec in accordance with your wishes.
And to tie the two issues together, I would suppose that if you are copying logs to syslog then the above control over the journal will only address half the problem and you would need to check that syslog log rotation is correctly happening, and that appears to occur daily via a systemd timer and service.
Yes, I understood that this question was only a means to an end. Unfortunately I can’t provide any useful info on that.
All I can say is that I have had essentially zero occurrences of sudden power-off. Once or twice in six months I have failed to get on charge in time (an easy mistake to make) and it has either shut itself down (after issuing the warning messages and illuminating the blue LED) or it has just run out of power and died. I would not consider that unexpected behaviour though. 