great questions and lots of good info.
This willl most likely be a write and then turn off application.
Thumb drives may be the way for me.
If I were in your position, I would do the following:
I would purchase 2 ea. 2TB external units. Any well known "Brand." Your usage description means USB 3.0 would be nice, but not worth a lot of extra money in your environment. I would only plug in (and turn on) one drive at a time. Write your files to the drive. Swap drives and do it again.
Reasoning: USB (Thumb drives ) would be much more expensive to get the same amount of storage and backup. USB (Thumb) drives are still susceptible to failure (thermal, shock, solderjoints, etc.) If you follow the plan above, the likelyhood of losing your data is almost zero. Your MAIN
computer system drive AND BOTH of the external drives would all have to fail at the same time. While that could happen due to theft, fire, or some other natural disaster, even that could be reduced if you always kept one of the portable drives at an alternate location (family or friend.) Write data to one and then swap the devices when you can and write the other.
2 "cheap" drives, used like that, would be better than one Expensive drive; however, 2 medium priced drives would be better INHO.
Hope this helps.