Here's a refined version that keeps the intensity and depth of your critique:
Oh, and don’t even get me started on the secondary characters! The only ones worth anything are Anne and the youngest son. Anne is refreshingly decent, and the youngest son—mischievous as he is—actually has heart, wears his emotions openly, and can see people for who they really are. We even get to see him grow from a spoiled brat into someone with a real sense of responsibility. But the rest of the duke's family, starting with the duke and his other two sons, along with practically everyone else in the kingdom (especially the so-called "holy" kingdom), are all garbage. They treat her like a doll, never say what needs to be said, and just leave everything hanging. How on earth is she supposed to know their intentions if they never speak up or show her any real respect?
Honestly, this entire manga—and nearly every character in it—feels useless. The author should either change the plot entirely or end it. At this point, I’m not just frustrated with the author but the entire production team. Creating a manga involves more than one person, and I can't believe no one on that team has spoken up to say, “Hey, this isn’t working!” It’s almost painful to read; each chapter just drains a little more joy and hope from me.