Hi Anthony,
There is a bug in Central where tmp files are not deleted. It will be fixed in the next release, but see Errors with sending forms to ODK Central because hard drive is full for a workaround that we shared a month ago.
In your attempt to resolve the tmp issue, you ran docker-compose down which is unfortunate because it will reset your database. We warn about the dangers of running this command at https://docs.getodk.org/central-install and provide recovery instructions at https://docs.getodk.org/central-troubleshooting/#troubleshooting-docker-compose-down. Try the recovery steps and report back with what you find.
As to the backup restore, that does seem like a bug and I'll ask the team to look into it. Before we force anything, have you successfully restored backups of less than 10GB before?
Yaw