A more “back of the envelope“ approach to handling uncertainty for this sort of thing is the “three point estimate” approach, where you give a best, worst, and most likely estimate for each sub task. These points are implicitly used to parametrise a distribution, and then you analytically find the overall uncertainty, rather than through simulation.
We used a different three point system. 1 point means we know exactly how to accomplish this work. 2 means there is uncertainty, but we are confident it will completed. 3 means there are unknowns that require more digging. A 3 pointer is a good candidate for breaking the story/task up into smaller chunks.
