The fix Cass route is essentially using their bond to hopefully heal her. As Elis didn't give up on her, even when she went way too far with the coup. And if Cass genuinely loves Elis, and maybe even her other siblings/cousins, she would be reasonable enough to change. If not for anyone else, at least she will try for them. It's not really exclusively a love route kind of thing. At least in my opinion. Since you don't really have to have a positive relationship with her to get this path. As far as I know. So it's more like healing Cass and then going the romance route, if you so choose. Or having her by your side, after Elis gets on the throne.
Now the break her route is different. Elis is basically trying to give her a twisted form of stockholm syndrome. Pretending like he's the only one that cares about her while having her cruelly tortured in her cell the entire time. And, it seemed to be working. Which is actually kind of surprising since Cass should've realized this is what he was attempting to do from the start. If she was as smart of a character as believed. But, if I remember right, Elis was the one that became too impatient and basically blew it, somewhat. But going forward, I would imagine she'll get more "correctional" treatment.
Personally, I feel like the break her path won't be as developed as the fix her one by the end. The objective there is to break her and turn her into your pet. One, that's getting kind of close to being considered dark for Patreon. Especially in these current times. Since there's going to have to be a lot of coercion, a bit of torture and pretty much rape. And two, it's because I don't think many players are actually going to play that path too much. Since the goal is to have her imprisoned and toyed with the entire thing. That's pretty much all there could be there. At least from what can be guessed at this current time.