The extra info is great, thanks for that. The only options I can suggest are:
(1) Use a custom column or tag to track which books you have processed instead (that is what I do, a simple yes/no column). However that does rely on you editing from calibre to conveniently flip the yes/no after editing. And won't help you figure out the backlog.
(2) Roll your Python sleeves up and write a simple plugin to do it for you. It could populate a custom column when run with the latest timestamp.
(3) Perhaps chaley's template language for custom columns would support something like this. I would be surprised if it did, because it would be intolerably slow everytime you started calibre with a big library like yours. But I've never really looked into that language since my own column needs are too simple. Again it requires some head-scratching/coding thinking, though chaley is very good at helping out. Certainly if he spots this thread he can say whether this option is even possible.
If it was me and I was in the situation of already having thousands of books "done" already without a custom column, I would write a plugin for it. But then I've already been through that Python/plugin learning curve...
