Hello, books must have a storyline or universe or characters in common to be a series. "Universe" means the setting. Some of the characters could encounter each other, or, some of the characters are the same in each book. A shared theme is not a series by Goodreads definition. Can you please tell us how this series meets the definition of a series by Goodreads standards? It is necessary we know this information before creating a series, not following the rules can get our librarian rights removed, by looking at amazon it seems this series does not meet the criteria one reviewer pointer out "This is the second book in Susan Fanetti’s Crossings Collection, a series of standalones connected through their focus on Transatlantic love stories." I believe "Transatlantic love stories" would be a theme thus not meeting Goodreads definition.
https://www.goodreads.com/book/show/5...
https://www.goodreads.com/book/show/5...
https://www.goodreads.com/book/show/6...