I do agree that while I was hoping that the Dark Queen would have a more fulfilling ending, I still think she was able to achieve this change.
As you said, "Change doesn't have to be something massive, changing little things is still change," and for her, she's stated multiple times that she was fated to be the Dark Queen and that nothing will change this. She even tries to get the hero to accept this "truth," but as time goes on, she slowly starts to falter, to the point she questions why the hero is so confident she can be defeated. At this moment, she's already questioning what she believes to be indisputable.
She has also mentioned that there have been others who have tried to bring her down, and still, she's never had to put in any effort. But in the last battle with the hero, she is seen giving her all, something she probably has never done before, which is another change that she experiences that disrupts her normal routine of things.
In her last moments, as she's coming to terms with the fact that her end is near, and it's thanks to the hero, she can accept her death. I think the fact that the hero was able to change her mind about having to stay with this undying, unforgiving tyrant is the change she needed , and while bittersweet, I think the ending was very fitting.