That’s why Yuu is way too good than her. Unlike Sayaka who just waited, Yuu took action genuinely for Touko’s sake. Yuu didn’t cow to step up and extended her hands to the “person” that Touko tried to hid first even they only met for months. Yuu accepted all her contradictions. Yuu accepted who Touko is— both sides. Everything of her. Even though, this is what Sayaka is up doing now, she was late.
You almost sound like you think of Sayaka as an opportunist. Let's not disregard the fact that Sayaka has been supporting Touko all along in her goal of becoming her perfect self. She admires Touko's perseverance in achieving her ideals, that's what she loves about her (Ch. 12) and in that, she's supported her love amply. Everyone has their own opinion of what's best for their loved ones, Sayaka simply had hers.
Her only major flaw I find worth mentioning is that she was reluctant to pry into Touko's personal matters, in fear of ruining their relationship and never considering what lies past that initial turbulence might be something better for both of them. However, whatever she found out about Touko on her own, she did accept. She didn't just fall out of love after learning of Touko's meek and unreliable side. She doesn't seem to know Touko hates herself (I doubt she'd endorse that mindset if she'd known). How would she? Touko on her own, never really confided her inner struggles to Sayaka, like she did with Yuu, though that may be a result of her enabling Touko for so long.
I'm not trying to dispute Yuu's pro activeness in her relationship with Touko as she did do a lot to bring Touko out of the dark and it's what makes her our most beloved character in the manga. But as I see it, Yuu's affection for Touko has always been gradual in its development. During first few months, she put up with Touko's every antics because she's highly tolerant and later joined the stuco and helped Touko out of her kindness, and grew fond of her with each passing day until she was nearly certain the emotion that'd been growing inside her heart was indeed love (Ch. 32). So we can't really say one was "late" than the other IMO. "Accepting everything of Touko" has different meanings for Sayaka and Yuu.