Hi. As you are deeply thinking about these sorts of things, I want to see what you think of my https://useorganizer.com which turns the todo idea on its head a bit by being timeline oriented.
Seems very cool! I like the concept. I am still designing a "project" sort of organization primitive myself, which would incorporate more time based views of progress.
Just from browsing around and playing with it a bit, it seems there's still a lot of polish needing to be done. Quite a few areas where data is leaking across UI boundaries and some of the hard problems like data management and retention are stubbed (export is nice but it's the floor imo)