That last image. Frank has made peace with his life and his loves. He knows that with Brianna, he finally got love right. And now he's seeing off his daughter to save her mother and her other father. He's seeing her off on the docks as he once saw off Claire in the railway station, in wartime, or to medical school, at a bastion of the establishment in the 1950's.
Frank knows now, for sure and at last, that Claire was right: no amount of time, and nothing that he could do, would win her back from Jamie. He got that wrong. And he knows that he'd choose her again, anyway: her and Brianna;