I think a good general surgeon and a dentist going back one thousand years could make a big impact on society, but you’d have to come up with an anesthesia. Aspirin would be a powerful drug if you could find how to make it. With a little bit of time and effort you could probably come up with a simple antibiotic. I’m fascinated by the lack of simple wound care skills. You could use placenta for skin grafting. How did people deal with kidney stones? You’d really have to come up with some powerful analgesics but I imagine opium and cocaine were used long before then.
Aspirin would not be hard. You can extract insulin with 1900 kitchen tech, but you need hypo needles too.
Lots of drugs we use today started as plant extracts.
I agree, if you were careful not to step on toes or get branded a witch (often the same thing) you could go far.