OK, instead of increasing the disk space or trying to hunt down files to delete, I decided to set up the new server on a temporary domain to test everything out (and might just migrate to that domain permanently).
One issue I ran into was Cloudron wouldn't recognize the DNS in the domain settings when changing the dashboard domain. Since I set up the server with IPv6 I had to enter both the IPv4 and 6 addresses into the DNS config for my domain (I'm doing it manually since Godaddy and NameCheap won't let you use their API unless you have a LOT of domains and/or pay a premium). I'll have to do that for every app individually.
I'll have to think ahead as far as disk space is concerned so I don't run into the Postgresql problem again.
Thanks for your help!