Rather than periodically running journalctl --vacuum-size=500 to free up space, you can just limit the journal by adding the following to a new file such as /etc/systemd/journald.conf.d/size.conf:
This will limit the journal from using more than 512MB. That said, if journald is filling up fast, then something is spamming your logs and you could run journalctl -a -f to get a sense of what is being written to your logs.