There are a few possible approaches. I figured they're using b2.
I'm pretty sure Synology has builtin tools to mirror to it. Backblaze will backup external drives that are attached to the computer, but if they are disconnected for 30+ days the data is deleted. While network drives aren't backed up there are ways to have them appear as local drives (which I hear are a pain to deal with).
Yes exactly, this is what I'm using. There is a time machine folder on the NAS, the Synology tool mirrors that encrypted folder to backblaze. I have the backblaze sync set to run at 1am so it's not uploading and affecting my bandwidth while I'm (typically) awake. Yes, my remote backup is up to 24 hours behind my local time machine backup, but this is acceptable to me since it's only for catastrophic recovery.
