Configuring journald.service to log its own debug_log messages
Run systemctl edit systemd-journald to add the environment variable:
[Service]
Environment=SYSTEMD_LOG_LEVEL=debug
Save and exit, then restart the service.
Note that unlike other systemd services, journald's own messages will not go to the journal – they will be logged to the kernel log buffer, i.e. dmesg (and will not be imported from there back into the journal, either).