He was even a bad plot device I'd say. Because in the end, Yoshino didn't become more clear about his insecurities or boundaries, it was Rou who realized he messed up because of his friend's message and Niko. Yoshino having no agency at all was the worst aspect of this manga. If this were a Niko spin-off with a Rou x Yoshino extra on the side, it'd have been genuinely better because the author clearly was more interested in drawing Niko than developing the first couple imo
That's it! I mean, if the idea was to give more depth, more drama to the story, there were other ways. Rou could be busy because of work and that could create some distance between them. But a jealousy plot that doesn't serve any real purpose for the couple, not even to bring them closer, but rather an attempt to show the "growth" of a character. If the author just wanted to draw the character, maybe they could do what you said, a spin-off with Niko as a new main character.
Introduce a new character for what? For a moment I thought Niko would become a love interest for Sui, but he was just a plot device. I would have loved it if it was just Yoshino and Rou being happy and cute together in the second volume.