Quick thought, without any form of pruning, the logs may well just grow and eventually fill the SD card. Worth keeping an eye on. It partly depends on the default settings used when journalctl was compiled.
This discussion will show how to check the defaults emonSD next steps: log2ram - #29 by borpin, and this how to change the max file size systemd/Journal - ArchWiki. Best way it to create a drop-in so future system updates will not overwrite your settings.
The problem with this is the /val/log tree is mounted to tempfs (as you found out). @djh method uses the power of the system in that leaving the setting for storage at auto, journalctl checks for the folder /var/log/journal at boot and if it finds it, uses persistent storage.
However, it is better to create a drop-in for changes to configs and check on the max file size & number of files combo.
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By default, SystemMaxUse= is 10% of partition. SystemMaxFileSize= defaults to 1/8th of SystemMaxUse=. However, I’m not convinced that these files will automatically be rotated as the max number of files defaults to 100!