I've read an interview with Gregory Lewis on 80000 hours. He argued that due to counterfactual, doctors don't make a big difference because doctors are already highly competitive, so you don't make big impact especially if you work in rich countries. There's a problem: You can still make a difference by being a better and more patient doctor. I don't know the doctors in America, but in East Asia, not every doctors are good. Some just want to make money and they treat the patient poorly, making them suffer more.(such as misdiagnosis) So, if you can be a good doctor, the counterfactual case would be" You replace a worse doctor than you". I don't know how valuable would it be, but this shows doctors in rich countries may be more altruistic than normal careers. Also, you need to consider how prolonging one's lifespan influences his/her relatives' suffering. Some people feel desperate if their relatives pass away. Being a biology researcher may be more valuable than just being a clinical doctor in the long run, but, I think we may underestimate a doctor's impact.
I think it is potentially difficult to determine how good the average doctor is in a particular place and how much better one would be than the average, but if one could be reasonably confident that they could make a large counterfactual impact on patient outcomes, the impact could be significant. The easiest way to be sure of these factors that I can think of would be to go somewhere with a well-documented shortage of good doctors, while trying to learn about and emulate the attributes of good doctors.
Being a doctor may not be one of the highest impact career paths on Earth, but it might be the highest impact and/or the most fulfilling for a particular person. High impact and personal fit/fulfillment are fairly highly correlated, I think, and it's worth exploring a variety of career options in an efficient way while making those decisions. In my experience, it can be very difficult to know what one's best path is, but the things that have helped me the most so far are experiences that let me get a taste for the day-to-day in a role, as well as talking to people who are already established in one's prospective paths.