6$ / TB / month is a fool's bargain even for something as low as 10 TB. One can buy a used LTO-6 drive for a few hundred bucks and build tape libraries that span hundreds of TBs.
There's no Cloud-based backup service that's competive with tape.
Plus restic or borg or similar. I tried natively pushing from truenas for a while and it's just slow and unreliable (particularly when it comes to trying to bus out active datasets) and rsync encryption is janky. Restic is built for this kind of archival task. You'll never get hit with surprise bills for storing billions of small files.
What does Backblaze's backup software have to do with B2? Backblaze B2 is just storage that exposes the same API as S3. You can use any backup software that supports S3 as a target.