I use rclone, not restic, to back up to backblaze. So maybe my advice is not relevant to restic. But what I’m going to say isn’t touched on in other comments, so I’ll put it out there.

The rclone docs say for Backblaze in particular, users should increase the number of concurrent file transfers because rclone’s default (4 concurrent transfers) is too slow on Backblaze. I don’t see this mentioned in other rclone docs pages for other providers (of the ones I’ve looked at: Jottacloud, Overdrive, I forget what else). My impression is it’s something about Backblaze. Maybe its something you could look into.