@nebulon Okay, thanks. Importing the backup wasn't completing successfully since I'd had to use the Clean Backups function to free up space.
I ended up just restoring the last backup image of the whole Droplet from DigitalOcean. That put things back as they were the night before the disk filled up so I only lost items from a single day.
I was relieved to find that once I'd gotten things back to a green status I was able to complete the setup process for an external backup in an S3 object storage account. I tried setting up Backblaze B2, DigitalOcean Spaces, and Wasabi on Sunday and Monday, but they always errored out with a complaint about the bucket not being available to the region the DO Droplet is located in. Monday I was able to get my DigitalOcean volume mount set as external backup, but obviously don't want to use block storage for backups. Once I'd restored the droplet and resolved the space issue, though, I was able to add B2 as external backup storage with no issues.
Thanks for the help!