I didn't feel like that at all when watching the scene. We have seen Sayu plenty of times lamenting her situation or trying to encourage herself and Miuna. Kaname is a different story because he never thought about Sayu in that way before this scene, so it's unfair to say he would have needed some foreshadowing. But we have certainly seen him feeling lonely and left out, and getting beaten by Tsumugu to approach Chisaki.
I don't personally think the best outcome requires long years of suffering. The best outcome for Tsumugu would have been Chisaki accepting her new feelings during the five years. There was no particular reason preventing that either, since she did know Hikari loves Manaka. It's actually harder to understand why she didn't want to move forward and instead chose the limbo, well, aside from her personality which isn't the easiest and thus explains everything.