Ohhh boy, after so many people are suggesting I do simple files directly on the disks I went back and rethought some things. I think I’m landing on a solution that does everything and doesn’t require me to manually manage all these files:

• fd (and any number of other programs) can produce lists of files that have been modified since a given date.
• fpart can produce lists of files that add up to a given size.
• xorrisofs can accept lists of files to add to an iso

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So if I fd a list of new files (or don’t for the first backup), pipe them into fpart to chunk them up, and then pass these lists into xorrisofs to create ISOs, I’ve solved almost every problem.

• The disks have plain files and folders on them, no special software is needed to read them. My wife could connect a drive, pop the disk in, and the photos would be right there organized by folder.
• Incremental updates can be accomplished by keeping track of whenever the last backup was.
• The fpart lists are also a greppable index; I can use them to find particular files easily.

Downsides:

• Change detection is naive. Just mtime. Good enough?
• Renames will still produce new copies. Solution: don’t rename files. They’re organized well enough, stop messing with it.
• Deletions will be disregarded.
• There isn’t much rhyme or reason to how fpart splits up files. The first backup will be a bit chaotic. I don’t think I really care.

Honestly those downsides look quite tolerable given the benefits. Is there some software that will produce and track a checksum database?

Off to do some testing to make sure these things work like I think they do!