The large majority of the work code does can probably be boiled down to a series of transformations done by a hierarchy of pure functions with a fixed number of

The large majority of the work code does can probably be boiled down to a series of transformations done by a hierarchy of pure functions with a fixed number of inputs/dependencies, all of which are easily testable in isolation. It's unconstrained side effects and dependencies, resulting in an increase in complexity, that seem to cause the major issues and have to be managed. The real problem, of course, is the human capacity to comprehend (or not) the entirety of the system or subsystem by modeling it correctly in the brain.