Your background is in engineering so that informs your choices. Further, math/CS are not linearly enumerated fields. E.g., there is no field of CS, it is a doma

Your background is in engineering so that informs your choices. Further, math/CS are not linearly enumerated fields. E.g., there is no field of CS, it is a domain of many fields. Think tree, not list. That domain would take you a lifetime to get through all fields, even for an undergraduate level. There are now millions of people around the world and some are developing new material at the cutting edge. These are the reasons why degrees are now given in Systems. The only reason to try to learn all these areas is if you are designing a systems tool or the tool is you, yourself. Systems is a little appreciated area but one of increasing importance. My degree is in Computer Systems Engineering and it it is very enjoyable work...if you like massive puzzles and are a comprehensive abstract thinker about applied areas. I need to make this more concrete so here are a few touchstones. Look at the curriculum for schools that offer degrees in systems. Weed out the ones that let you create your own curriculum, those are the ones to choose for a school, but the are too ill-defined to help you set your future. You can also take a look at the Nobel winning economists. Their work touches on both theory and application. One of my favorites is Schelling's work on the neighborhood problem of integration. The demographic issues of income, race, neighborhood size and shape made it an intractable problem until he stepped in. He used a system approach of using image processing linear algebra to predict who moves in and out of a neighborhood to predict the changes across generations. He solved a problem that others could not and for that, among other contributions, he received the Nobel prize. His work is demonstrated in the free software tool Netlogo. It also lead to the work at NorthwesternU in complex adaptive systems analysis by Scott E Page. We see it also used in the demograpgic analysis work in the campagns of Obama, Hillery and Trump by Cambridge Analytical. The way I think about this discussion is that "All the easy problems have been solved, now we have to work on the difficult ones." And that will take the use of new tools that collaborate with us, and tell us which tools to use.