I hope you don't mind, I've moved this post into Brian's area, since he's the expert in these matters.
All I can suggest is to run through all the worst-case scenarios in your mind:
Fire or theft
Internal hard drive failure
NAS failure
Backblaze goes out of business or corrupts your files
All of those seem very unlikely, but they're all possible, so you need to be confident your system is robust enough to withstand any of them on any given day.
Absolutely not, because of Backblaze. https://www.damiensymonds.net/2012/02/to-dng-or-not-to-dng.html