Just to note, Hetzner’s lowest tier offers only 20GB, but that’s been working for me. It can be worth tidying up the backups directory before trying an upgrade: having, of course, taken a new backup and copied it elsewhere for safety. (It’s a small forum: a backup is 120MB.)
In another case, I use Digital Ocean’s lowest tier, offering 25G, and in that case the forum’s backup comes in at 640MB.)
The one time I had trouble, it was the journal thing, but I cut that right down:
# journalctl --disk-usage
Archived and active journals take up 104.0M in the file system.