[catchclose8919]

> the vast surface area of the project makes it hard to hold all of it in my head at once

...yep, in general one of the root causes of having to do this kind of nasty work was someone (maybe even your past self) having failed badly at decoupling a distributed system ...add some "documenting it was supposed to be milestone 3" but then priorities got restructured and it never happened, then people left and others came, and voila!