The middle part of a trilogy can be hard. The first part is usually written so it can stand alone. But when an author starts the second part of a trilogy they are already thinking about the third, so it has to be a connector. It continues the story and sets up the finale, but it, too, has to stand alone to keep people interested.
It can work. The Empire Strikes Back is the best of the original Star Wars trilogy. The Dragon Republic might not be the best of R. F. Kuang’s trilogy, but it more than delivers. And it does what the second part of any trilogy must do: it makes me want to pick up the third book immediately.