320 GB is prohibitive! Unless you are a major university and have been collecting student uploads for years. Whatever, the first thing is to make sure that you have a solid backup/recovery strategy. Whether through https://docs.moodle.org/en/Site_backup and https://docs.moodle.org/en/Site_restore or through the IT infrastructure (virtualization, etc.). What is your Moodle version? What additional plug-ins are installed? What are the system software (LAMP stack)? Hardware, dedicated server(s), VPSes, clustered, ?
You need to clarify whether those 320 GB are genuine. This is not easy due to the encrypted files names in the moodledata/filedir/. You might need a tool like the https://moodle.org/plugins/report_coursesize. A typical problem is people (teachers) taking course backups and leaving the backup in Moodle. Or, you might have to find out an archiving concept for very old courses.
Dumb question: You run the https://docs.moodle.org/en/Cron, right? What do you have under Site administration > Server > Cleanup?