I don't think Cain's story will be brought up more than it has. It's a feeling I have. Plus, we don't have many chapters left until the end tbh. Probs 5-10 chapters?
We all waited years for even one chapter explaining Cain's backstory and it was quite brief. So I highly doubt the author has any interest in adding more...
I mean it would be cool to know more about him as a person, but it seems less likely.

Yahwi can rot in hell for all I care about. He didn’t change, not one bit. Still as selfish and self centered as always… on the other hand, Cain is the best thing this manhwa created. He’s the only reason I keep reading this. I wish for his happiness with all my heart. And I’m happy to finally see more about him, his family and stuff… hopefully the story focuses more on him and his past life now, that’s the only way to save this manhwa and make it interesting again.