O Rio Onde a Lua Nasce (2021)

Both Goguryeo dramas. Goguryeo is rarely featured in kdramas if ever so I'm glad there's another one after so many years. Fans who will watch River Where the Moon Rises should check out the representative Goguryeo drama, Jumong.

Strong female lead with dual identities. Badass as a fighter, but also femine and super pretty in imperial attire.
In The Song of Glory, the female lead is the noble's family lost daughter. She's raised by assasin so she's a strong fighter with cold and blunt personality. She will become emperor's concubine later.

Quiet and gentle, Min Jae Yi is everything a good daughter of a prominent family should be. Engaged to the son of the Second State Councillor, Jae Yi knows that the only way to bring herself and her family honor is to accept her fate, which she does with grace and charm. Devoting herself to learning everything she needs to know to become a good wife and mother, Jae Yi dutifully prepares for the future. Sadly, destiny has other plans for her future. Four days before her wedding, Jae Yi’s family is murdered. With no one else to blame for the unexpected tragedy, Jae Yi soon finds herself the object of baseless and horrifying accusations. Blamed for the death of her family, Jae Yi might not have had much of a future at all, if fate hadn’t decided to intervene. Bringing the lonely crown prince, Lee Hwan into her life at just the right moment, Jae Yi is saved from a dismal fate, but her salvation comes at a price. Plagued by a mysterious curse, Lee Hwan agrees to help Jae Yi, but only if she agrees to help him break his curse in return. Unable to refuse such an offer, Jae Yi agrees to the prince’s terms, setting them both on an unexpected path toward love. But what chance does their love have of surviving when one of them is a former murder suspect and the other is a cursed prince?

Historical setting, both are Korean dramas. ML and FL have a strong bond and good chemistry. ML becomes traumatized by the constant war. Surprising hopeful/happy ending after ML is wounded/near death and thought to be dead/possibly dead by FL.

Both are historical dramas with a princess FL. Both MLs have suffered trauma and loss, and find their strength and healing in their love of the FL. Both dramas have surprising and unexpected happy endings of both MLs faking their deaths after a severe injury, and the FL going off with them to live a quiet life, despite being loosely based on tragic historical events.

Similar tone, historical setting, slow pace, and interesting characters. The ML's personality also seemed similar to me.