-Should I even consider a DIY postbacc given my currently low sGPA or should I look at doing an SMP/other master's program (either through doing a career intermission program or a break in military service)?
Your sGPA is a bit low but hopefully will rise with prereqs. At the moment, I would not pursue an SMP. Get through the prereqs, maybe some additional upper level bio / chem classes to show you can handle a science heavy medical school curriculum.
-Should I go back and take gen chem or should I try to enroll in higher-level chem classes like pchem or advanced organic?
Unless you received a C or below in gen chem, I would not retake it. I would not recommend p chem as it is one of the most challenging undergraduate courses will very little that will lend itself to med school. If you need to fix your GPA, look into genetics, cell bio, molecular bio, etc.
-Should I consider taking other science classes (higher level bio courses, A&P??) later on to raise the sGPA?
See above
-Will I need to take an intro psych class or will the upper level classes suffice?
Not necessary for the MCAT but some medical schools may have a social science requirement. My guess is upper level courses will be more than sufficient but check specific programs to be sure.
