Poking around the forum I found this thread on storage issues. I went around my server’s directories running the command sudo du -shc * | sort -h and found that /var/log/journal was gigantic 
I used the advice from this website on cleaning the journal, and ran this command:
sudo journalctl --vacuum-size=100M
This freed me 2 GB of space, and after doing so I was able to successfully run the migration of PostgreSQL from 13 to 15. 
But I’m still a little concerned there’s some remnants of previously-deleted apps (I installed and later uninstalled mobilizon and yet found a bunch of mobilizon stuff still on disk! 
). Is there a way to check that the database has been purged of any deleted apps? Ty!