I'll do you one better. It's not Jiang He the one that goes back, but Xu Qing. It's not a series end, it's a plot hook for a sequel!In a twist: Jiang He is suddenly sent back in time, leaving Xu Qing and the twins without a wife and mother. Series end.
He manages to use the geography to find her old home and her gang. Thanks to his knowledge of the future, he manages to leave something that will:
A) Survive over 1000 years.
B) Be something that Jiang He and the Twins can find.
I can totally see his father being the first one to notice that some events in books begin to change and describe a gang of people led by someone with the same name and description of his son.
Then a new old book appears, which narrates this story as a fantasy, and Jiang He manages to decipher that the altered situations are all around the places where Xu Qing hid something.
Those places have very very old paper, sealed in wax and honey to avoid rotting.
In modern characters, they're Xu Qing's writing across his life in the past, explaining to his family what happened.