One of those shows that just got better and better for me as it went on. The movie (The First Kiss That Never Ends) feels like great payoff for all the development between Kaguya and Shirogane.
On Crunchyroll the movie is available split into 4 episodes, as it was aired on Japanese TV I think, and listed as another "season". That's maybe why the other commenter thought that you've seen it already if you finished the series.