What's your filesystem? What is your storage setup currently? Can you get to a shell on the running OS?
You might want to boot to a live linux usb/disk/ISO mount your filesystem/drive that is full and delete your cached files from that.
Here's how to clear tables on a regular basis https://lemmy.world/post/207421?scrollToComments=true
Here's how to clear your cache in your postgres DB https://stackoverflow.com/questions/68816441/clearing-cache-in-postgresql
I would strongly suggest standing up your self hosted instance in a docker container if you haven't already
https://blog.colic.io/2023/07/07/self-hosting-lemmy-a-step-by-step-guide-with-docker-compose/
Also now might be a good idea to look at grabbing a cheap external drive and backing up to that as a worst case.
https://join-lemmy.org/docs/administration/backup_and_restore.html