

Two similar panels
401 vs 430
Similar panels depicting different feelings and realities.
In panel 401, we see Ruka maturing. She is longing for something as she thinks of Kazuya, the guy she loves. She planned to take a new step forward—with him, and only him. There was both desire and love there. Even if her feelings weren't fully reciprocated emotionally, the physical connection made it worth accepting the role of "second choice" just to have that.
Between 401 and 430, she lived in a sort of dream; she didn't get exactly what she wanted, yet she still made significant progress in their relationship. She came to believe that the "second choice" would become the "first choice," and eventually, the *only* one. She thought she was closer to his heart than ever before. She believed sex would bring them closer—that with every encounter, she would break down the walls around his heart until she completely took the place of her rival, Chizuru. It seemed like only a matter of time.
In panel 430, we see her after discovering that he remains deeply attached to her rival—the person he loves and has always loved. None of the steps she took ever truly threatened the rival's position as the one who held his heart, and likely no future step she could take would achieve that either. All the emotion and fantasy she had been living evaporated, leaving behind nothing but pure disillusionment. She never really moved forward; she could stir his body, but never his heart. Now, she is in much the same position as in the first panel, but with completely different expectations. She is about to answer a call from her rival, already knowing what is likely coming next.
I haven't read the spoilers for the next chapter—and I won't—but I’d say the end of Ruka’s one-sided relationship and her dream is imminent.