Unfortunately, as far as I know, the only way to reorder stories in lists after they've been added is to click and drag the items into their new position. With 500+ poems, that seems a daunting task! Maybe contact Medium support and see if they have any better suggestions? Sorry I can't help more, but I'm unaware of any feature that would auto-reorder by date published. You should suggest that to Medium as a feature to add in an update.