I think he was intrigued after their wedding night once he saw she had totally changed from the type of girls he had always hated. And then it was further developed even though not in a good way (like when he wanted to force himself onto her when she met Lu Yihan for dinner), and after she miscarried it was obvious he wanted him to be a part of her life (when he thought they could have another baby if she was willing but at this point he thought Su Qianci was in love with Lu Yihan)
Even though I've stopped reading raws due to time constraints and my workloads, I'm keeping my faith in Li Sicheng when the next troubles come. And although Su Qianci so far not being too proactive even though it's her second lifetime I couldn't blame her. Like you have stated, she wanted to live a peaceful life and get a divorce from Li Sicheng. She herself didn't know her love for him would be reciprocated. And for fans who have different opinions regarding her not being smart enough, please give herself some credit too. She's not quick witted and strong like Ye Wanwan but she indeed has qualities too