I'm currently developing the lesson plan for a large (~100 students) survey course in history for the fall semester. Since this is my first time as instructor of record for the course I'm having some problems balancing how I would like to teach the course (several books worth of reading) with feedback from others in the department (i.e., limit the reading). My concern is that while the textbook is quite solid, there are some significant gaps that I think should be addressed. How can I balance the reading for the course so that I don't overwhelm the students in my course?
Survey courses are generally designed to be superficial. The music department offers music appreciation. The fine arts department offers art appreciation. These courses expose you to the major themes of the field for the sake of appreciation and not mastery. The goal is to expose none experts to a field in order to broaden their horizon.
You state yourself that the majority of the students are not history majors. Therefore, why would you treat them like history majors and expect them to develop a deep understanding of military history? Few of us become experts in the electives and GEs we take as students.
If you are worried about the history majors you may want to include additional optional reading so that they can go deeper if they desire but weighing down STEM majors seems pointless. Better to stoke a fire/passion for history rather than burn them up with heavy content.
My undergraduate history courses were probably not anything considered typical at most universities. The reading load was usually a book per week, and the "course text" (when one was assigned) was generally to be fit in on top of the weekly assignments. I got used to it as part of the requirements for studying history at that school and dealt with it.
That said, such a workload is not typical, and there is the need to balance an assigned textbook with supplementary readings. The question is how much you want to focus on the main text. In general, "outside" readings tend to be much more interesting and valuable than the basic text—although you need to understand the basic material to derive greater meaning from the more specialized readings.
So perhaps the way to balance it is to carefully consider which passages in the main textbook are critical, and which ones are ancillary to the course contents, and fill in gaps or extend the work with limited supplemental readings. You can certainly have "suggested additional readings" to help people understand things that you think are worth knowing, but not required to know to be evaluated for the course.
How can I balance the reading for the course so that I don't overwhelm the students in my course?
Select some portions of the textbook to exclude from the reading assignments.
For the supplementary sources, select portions, that is, don't assign the whole thing unless it's short.
Supplement as needed in lecture.
The standard rule of thumb is 2 hours of study time for each hour of lecture.