USB RAID: good for back up ?

Is this important data or just digital hording? That's probably the most important aspect to think about. It sounds to me like it's the latter. Stuff that you like to have but if it was lost, it's not the family photos our other irreplaceable data.

For that USB raid, or even more is drives are fine. The problem may be how often do you want access. If it's something constantly available then maybe a small home nas might be a better option.

I don't have any recommendations for a product, but something to be cautious about with things like raid 5/6. Cheap appliances are cheap for a reason, and double/triple drive failures are known to happen. Spinning rust is pretty cheap, so it might be something you want to keep multiple copies of sitting on the shelf.

As long as you have an offline copy of this data that can be used to fully restore the data on your system you are fine. A proper backup is a copy of the data that can be restored if the system or network the hot data is on completely destroyed that enables it to be completely restored data wise.

In terms of RAID, that is not a backup. RAID1,5,6,10, etc. is to help keep your system online in case of a disk failure to help give you time to replace failed disks with some options giving you increased disk read or and write performance.

I would recommend you keep something offline that you backup the data on the USB HD from time to time. Something that can do versioning would be ideal so you can view the differences over time.