I think if anything, Sian should end up with Yuk. Jeongsu is a red flag, anyone can see that. Including Sian, I'm pretty sure. And like you said, Sian isn't reciprocating Theo's energy (which sucks because I shipped them at the start).
I don't exactly love Yuk, but Sian is the only girl he genuinely made an effort to approach. Like, he was actively playing with her in the pool, and then he made his feelings for her clear. He hasn't done that with anyone else. He was really vague with Minseol, but he made his feelings clear with Sian.