[sn9]

This is a common complaint with a simple solution.

Just look at the curricula for various undergrad math programs at some number of decent universities. They have a set of required courses and the course descriptions usually list the prerequisites, and sometimes this is even accompanied with a diagram of the DAG as a flowchart.

For any given course, there's usually a canonical text or set of roughly equivalent in quality texts.

You can do the same thing for basically any area of study that can be found as an undergrad major at most universities.