I use RAID myself for backups, to get large enough volumes. That doesn't address a box failure - if the NAS fails and destroys the drives inside, you've lost both primary and backups. Even if using RAID-5 (like I do), if a drive fails, followed by a 2nd drive failure during the rebuild, you have lost all data.
Hence why I don't use my NAS for backups, and the files being backed up.
I've long heard good things about Backblaze, but my ISP upload speed isn't good enough. A common problem in the US, where we lack meaningful ISP competition and enjoy the resulting high prices.
If your upload speed is good enough for your data size, excellent. I still wouldn't put primary and backup data in the same place. Not even the same cloud service. Even a data center can burn down - Iran hit at least one Amazon data center in the middle east recently. Amazon estimates months to fully repair everything, and I don't know if all data will be recovered.