Our Son is a Grinnell alumni. Our daughter has made her way to medical school via LAC (Vassar) > Masters of Biologic Science (Tufts) > 2 Med school acceptances. I highly HIGHLY suggest you do the major you are passionate about and if you are interested in medicine also take the 12(14?) core classes you need for medical school. As you progress you will find your path. Schools don’t need to see a Biology, Chemistry, Biochem major. Looking at medical school as an incoming freshman is such a long a winding road that it is a bit overwhelming and run the risk of the path causing you to lose focus. Learn stuff and make sure some is core to medicine. Only the old school MedSch really care about the major. They want MCAT scores, activities that show med passion, and grades.
Medical schools are more and more (not all - need to know your home state school requirements) looking for well rounded LAC type students who can also do science. Grinnell can do this very well. As you progress down the path keep an eye on what your home state schools require. Sure it is nice to think about the top private medical schools and aiming for that is great but the easiest acceptance is always at home. They are also looking for students that are a bit older. 23-26 is actually a plus. If you agree with that then the Masters degrees are just a chance to enhance your MCAT with more knowledge (better schools and acceptance %), volunteerism, and medical exposure.
I am no expert in this area but having gone down this path twice I can say you need to take this one small step at a time.