For these particular files I do also use Backblaze, that's definitely useful if a subset of files were lost but having to download an entire 8TB hdd worth of data would be pretty painful with Backblaze (their physical hdd shipping isn't really a good option as I'm not US based).
In this case as the external drives were on offer I bought two of them with the intention of the second drive to be the backup. The files here aren't really really important, but would be very annoying if I lost it all.
For my truly critical files (things like family photos, secure documents etc) I have other multiple levels of backup, both with synced cloud services and local backups just to be safe. Backups upon Backups upon Backups there! (3 - 2 - 1 and then some)
Just to answer your question:
It's quite a lot of data but not insanely important, it would be really inconvenient if I lost it but far from the end of the world. I'm looking at local backup because in the event of an entire hardware drive failure having to download all the data again over the internet would take an absolute age. Not that a cloud backup isn't also good (I am using backblaze) in case the local backup drive also failed at the same time.
I do however have other things that are really important which I already backup with multiple levels of cloud and local storage (such as family photos, secure documents etc) - I will check out tarsnap as well though as it sounds interesting Thank you