Mathematics is definitely a weakness I feel acutely as I press up against the boundary between knowing-about and knowing-how-to-do.
I'd say the next book to read in a formal direction is Hartmut Bossel's Systems and Models, which happily dives into the calculus. Then, as my sibling comment explains, control theory, signal processing and information theory are major influences on various branches of systems thought (systems dynamics, cybernetics and so on).
That said, as Sterman points out, many interesting problems can't be solved analytically and require simulation in order to come up with useful approximations. Furthermore a page of sigils is comprehensible to relatively few people: a systems model is useful only insofar as people will believe its descriptive, predictive and prescriptive qualities. If they don't understand it, they won't trust it and will ultimately ignore it.
Systems dynamics and other simulation approaches (particularly discrete event simulation and agent-based modelling) are much easier to explain and verify qualitatively with subject-matter experts and end users.
Definitely read Sterman first, nobody else has such a thorough treatment of the field.