how to make thumbnail cache permanent?

I'm trying to thumbnail an entire image directory consisting of upwards of 100,000 files, a thousand per sub-directory, to minimize delays displaying the thumbnails for each directory. (Making it worse, some take quite some time to compute due to interspersed mp4 files which have been extracted from "picture with motion capture" on my phone). Over time I have noticed that my thumbnails have been mysteriously disappearing which is really frustrating as it takes hours of going to each directory and waiting for all thumbnails to be generated before going to the next one, and so forth.

Recently I discovered that I need to tweak the cache settings to prevent this. I used gconf-editor and in /desktop/gnome/thumbnail_cache I set both maximum_age to -1 (never age out) and maximum size to -1 (no limit). After that I started opening each directory and waiting for it to thumbnail and then on to the next one. I was monitoring the process when suddenly even while thumbnailing was in progress over 15,000 thumbnails suddenly vanished!

~/.cache/thumbnails$ ls large/ | wc -l
23021
[about ten minutes later]
~/.cache/thumbnails$ ls large/ | wc -l
7801

Now not only has a bunch of time and flash erase cycles been wasted for nothing, but I can no longer trust my system as apparently at any time, even a few hours after creating the thumbnails it can arbitrary delete them for unknown reasons.

How can I ensure that my thumbnails are permanent and never erased like this?

I have read that dconf-editor is another way to edit these settings, but it is harder to use, offering no hierarchy view, and further there is nothing under /desktop/gnome except "crypto" so I have no idea what the path to thumbnail settings would be or it would even offer anything that gconf-editor does not.