The post-bacc's only function is to show whether you can handle the prerequisites for medical school, so it is commonly used by career changers or people who were focused on something else outside of medical school. So in your case, you can finish your prereqs after you graduate without any worries and not having to rush through.
Whether you take prereqs as an undergrad or after you graduate is irrelevant. Just make sure you do well.
My cGPA is a 3.4; I will admit that I had to retake my inorganic series, but was due to a serious medical illness that required a withdrawal from an entire quarter and a hospital stay, I retook the entire series and attained B's in it. A's in physics, A's and B's in everything else. I'm a reinventor btw, had a bumpy past but am showing a stalwart upward trend now...have not taken my mcat. I'm at a loss as to whether I need to just finish my BA now or just keep chugging along and do a science series every year. Organic this year, Biology my final, with some calc on the side.