USB flash drives generally don't have good lifetime unless they are industry graded, so I would not trust them as only backup option for storing bitcoin keys.
I'm not sure why the OP isn't considering USB external hard drives for backup purposes, because HDDs have a much better lifespan than the flash that you can find in a USB pen drive.
I know it may sound overkill based on the storage size, but you can easily bring the price down by buying like a 128GB drive. Or maybe even 64GB drives. They seem to be available on Amazon when I checked it.