What exactly is the risk of leaving a small rectovaginal fistula alone?will surgical repair cause more complications w/thin septum from prior lar/tme?

See a specialist: Someone with a lot of experience with fistula repair, as they can be difficult. But leaving it alone can cause recurrent infections from stool getting into the vagina.

Chronic Infection: I think you should discuss this with an experienced surgeon, to fix the defect, otherwise not correcting it is more risky and complicated, leading you to more complicated major surgical procedures due to infections.

Best option treatmen: If it is left alone, stool will enter vagina, stool and flatus coming through will not pleasant, besides gives vaginal infections, surgical correction ( in a non malignant type ) will have excellent results and normal life , please consult a gynecological surgeon who is well versed to handle in this kind of problems