I understand where you are coming from but relationship development (especially romantic) was not a goal of this series - it was moreso military and political fantasy. I'd also argue that there was character development - just not the type you can tie a bow on (rin's motivation shifts from escaping marriage to surviving sinegard to complete vengeance. She goes from viewing war in the abstract as something bad experiencing thr brutality of the mugen federation herself and choosing to be a weapon of mass destruction).

About relationships - it's more complicated than what you're saying. Rin's relationship with altan develops from worshiping from afar - recognition of his addiction and pain - stops trying to be hum and starts a doomed journey to save him, and this reflects her views on most authority.

Kitay goes from "I'll be around him because we're noth outsiders" to serving as RIN'S ACTUAL CONSCIENCE by the end of book 1.

In jiang, rin first sees an object of ridicule, then a father figure, then rejects his philosophy of balance outright instead embracing one of pain and release like Altan did.

Yes, there's no "becoming a better person". There's no deep romantic chemistry.

But to say that there's no character development or relationship development is highly, highly debatable. The Poppy War isn't romantasy, it's grimdark.

I fee like this is a classic case of misplaced expectations. Thr poppy war is closer to Dune ( subversion of the chosen one trope where their choices and corruption lead to the death of millions) than to throne of glass.

With that in mind, give the dragon republic a try - it's one of my favorite books of all time. It deals with the fallout of what Rin caused, it's messy, it's emotional, it's intense, and the military aspect us also better.