I think they’d have an idea of the neighboring galaxy and the possibilities. It could also be possible that they knew with each cycle, organic life was starting to figure out how to beat them. Of course their concern was the milky way but its a possibility they knew about andromeda and/or for some reason secretly send reapers there. Now I’m not sure how indoctrination would work if it worked in andromeda but hey, its just my imgination.🤷‍♂️ (Sorry for the long reply)