til

du -cha --max-depth=1 / | grep -E "M|G"

From this StackOverflow answer - ncdu is also a good tool for this, allegedly, but its output didn't really seem to have any correlation to the actual folders on my server. I feel like I have to do this once a year because of storage issues and always forget, so hopefully this serves as a reminder going forward. Turns out snap is the culprit, as per usual. In a fit of pique I uninstalled snap and all of its files, and lo and behold I'm able to finally update Ghost!