Sorry I didn't answer your question earlier - I was a little riled up. 🙄
I think it is great that you want to do research. You can join an MSTP after a year or two of med school, but it sounds like that is not what you are looking for. Personally, I don't think the "Ph.D." part of research training is all that useful (Q and anybody else please feel free to disagree). Research training takes time at the bench, giving presentations, writing papers. You don't need all these stupid classes, thesis committee meetings, TA for stupid labs, and then writing a dissertation. You will want things to be as efficient as possible. Becoming a physician takes all your energy and passion. Becoming a scientist takes all your energy and passion. Trying to both well feels like a superhuman thing to attempt, and distractions need to be really minimal.
So if I were in your shoes, I would take a path where I do med school and pretty much focus on that except for maybe summers and you could attend seminars in your area of interest. Then look into a research track residency where you can short track your clinical training and do about 3-4 years of dedicated postdoctoral work. At the end of this time, you should be able to get a KO8 or similar NIH award (that's an NIH grant for young investigators). One of the big advantages of doing an MSTP is the no debt, but since there is such a shortage of physician scientists right now the NIH has debt repayment programs for MDs pursuing research careers.
I hope that helps and please let me know if you want to discuss it more. I am very interested in the whole MSTP vs separate MD/PhD vs MD/research postdoc route. Also, you may want to post your question in the MD/PhD forum.