These types of designs work when the problem is well defined. For something like this, where the easy part is stating how it should behave, and you're 100% sure

These types of designs work when the problem is well defined. For something like this, where the easy part is stating how it should behave, and you're 100% sure what perfect behaviour looks like, then by all means diagram and math it out until the system is complete or at least as complete as it can be for you to start performance testing some of the subsegments that can't be determined from first principles. But most people are not programming up things like this. Most of the time, either the data or customer usage informs next design decisions. Even so, I spend about 40% of my time these days in "plan mode" because nailing a plan always halves the time it takes to implement some feature and can even tenth it.