Find the current Disk Usage by journalctl journalctl --disk-usage

The safest way to remove unessesary entries is via the (means leave this amout left)
journalctl --vacuum-size=512M

After that you can verify if everything is still intact: journalctl --verify

Or as generell setting in /etc/systemd/journald.conf

SystemMaxUse=512M

different approach:

SystemMaxFileSize=12M
SystemMaxFiles=10

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