Agree with @dy003 about not retaking courses where you earned a C or higher since that’s technically passing. I’m about to start my second year as a post-bacc at UC Berkeley Extension, which is very much a DIY program. That’s one of the reasons I decided to go there - I got A’s in both semesters of bio when I took it at UDC and UCBx didn’t make me retake it in house. I’ve heard that if one wants to retake a lower division pre-req in which they earned a C or B to get an A, taking an upper level science class is a better move.
An MPH (or other science/health related) program shows adcoms you can handle graduate level work provided you get strong grades. Often adcoms look at upward trends which could work in your favor if your MPH grades are strong. A background in public health is helpful in medicine, especially for primary care and community health. Or if you decide not to do the post-bacc, an MPH opens doors to a lot of different jobs (speaking as someone who has a parent with an MPH)