Thanks again. I really appreciate your feedback.

My plan is indeed to have several backups, but also to enhance them with ECC.
This way, each backup has a better standalone value in the sense that it may be possible to recover small errors. From my experience, disks tend to break random sectors, corrupting one image (1-4 Mb) somewhere in the middle. If a whole image fails, then there will hopefully be another backup which has it. This can be automated via some merging script much like using checksum. Same goes for a complete disk failure.

The cost of this approach is 50% increase in the data size which is reasonable here. To my understanding, while disks (both magnetic and optical) do use ECC already, they will not use a 50% redundancy. For me (and surely many other photographers) the value of the data is worth this price, and even more. Especially with the low cost of storage nowadays.

Thanks as well for the link.