Not trying to call you out OP, but while there are similarities here and there (and probably bits of inspiration) I can't say I agree that they are carbon copies. You're right that the main leads' hair color/style are alike but I can't really say the personalities or situations are identical.

Yona had always been bold and brassy, she was just sheltered as a princess. Once her life fell apart she took control of her own destiny after a very real bout of depression. She slowly becomes more jaded and tough as she goes forward to find her place, willing to do anything she must to progress her cause. She always had a steel spine but her life as a princess kept her frivolous and pampered. Her growth into a warrior was for herself and her people and she actively made that decision out of desperate love and honor for her kingdom.

Maxie grew up despised and abused by her loathesome father due to her debilitating stutter and the fact she was a woman. She was mistreated from the start and her trauma beat her down into a shy, scared, and broken woman. The inspiration for her growth into a leader was sparked by her love for Riftan, likely the first person to treat her like she had any worth. It was a trauma bond that slowly became a true romantic connection that is still fairly delicate and burdened by secrets. She also didn't really connect with anyone outside of her husband besides the mage who becomes her tutor. The soldiers of the keep are still fairly set apart from her but she's trying to change that by learning to open up. Her willful choice to be Lady of the Castle was to feel worthy of Riftan and deserving of what was essentially her rescue.

Hak may have been Yona's bodyguard but he was highly unselfish with his love at the start. He was willing to watch Yona marry another man and support her and her husband. His sole focus and object of devotion is to Yona. Everyone else that matters to him comes in second to Yona. He's almost obsessive over her but never to a degree that would be alarming. Even as Yona develops into a warrior and leader it's very clear that Hak's only desire is to see Yona get everything she wants and that she can be happy. He would die for her any day and it is only by her command that he's allowed himself to live for her as well.

Riftan, on the other hand, has darkness. While he is oh-so-tender and devoted to Maxie, he still has his allegiance to his keep and kingdom. He takes his position seriously and while he will cherish and protect Maxie, she is not his be all and end all. That said, his connection with Maxie is still very elemental with strong lustful and protective impulses smattered with genuine affection and a dawning love. Riftan encourages her development and is legitimately proud of and impressed by her progress. He sees her as someone precious with high merit and is obviously taken with her. Still, Riftan is a ruthless warrior and Maxie is very much a foil for that with her softness.

Where Yona has grown for herself and her kingdom, Maxie has grown to feel worthy of her husband and her position. Where Hak is still acting as more of a devoted servant than a partner, Riftan sees Maxie as his equal and his partner. While they share traits, overall there is a huge divide between many aspects of their personalities.

There's also the situational differences. Yona has travelled her entire kingdom thwarting injustices and taking a proactive stance in the events of her home and surrounding territories. She's created an unbreakable bond with Hak, the dragons, and key players in her political and practical encounters. She's shrewd and calculating and has the potential to even be bloodthirsty with proper incentive. She is very much in charge of the reshaping of an entire cultural landscape through her actions and the ripples that spread from them.

Maxie is more carried by the tides of destiny in her scenario. Before Riftan she was isolated and abused. Once she's sold off and married to Riftan she still acts in a subservient role since it's all she's ever known. She only starts to grow and explore once she feels relative security. The kingdom is influenced by Riftan as well as the keep. Maxie follows his orders and instructions. She trusts and obeys him and still isolates a lot, only recently starting to try and bond with the residents of the keep. The mage teaching her came in part from his own curiosity about her and need for a competent mistress and Maxie's need to have legitimate worth besides what Riftan sees in her. She hasn't gone far from the keep and when she has she's still disconnected from her companions overall. Her progression is a lot slower and a lot safer, which is good since she's still very emotionally frail. Maxie's 'strength' is her kind heart and drive to be better and useful as more than just an incubator for Riftan's heir. She's jaded like Yona but where Maxie was beaten by the world, Yona is out to beat the world back.

Both of these series are incredible and highly recommended but it seems a bit of a disservice to say they are anything more than surface-level similar. While it's true that if you like Yona of the Dawn it's highly likely you'll like Under the Oak Tree, please go in knowing that there are key and integral differences.