Maybe I'm reading into this a bit much but I don't think Sage is gonna be a DIK-exclusive LI even later on. I know she gets extra points if you're a DIK but most of those points happen during earlier chapters, and I suspect the reason for that is she likes that you act similar to how Chad would in the early game.
As your relationship progresses, though, Sage starts to appreciate the MC for being himself rather than because he reminds her a bit of Chad.
Edit: Though if DPC does decide she should be DIK-exclusive, I guess I'm doing another playthrough