Yaaaaay I'm glad you're enjoying!! And congrats on Ending 3 LOL! 🔥 fellow flirt like me ayeeee I'm just kidding.
It's a little trolly but to get Sage:
SPOILER ⬇️
You need to play through a new game after Agreeing to the Plan in a previous playthrough. So this would have to be at least your 3rd "new" playthrough. But please do utilize the Skip function! It'll help you jump past all you already saw and get to the romance segments with Sage!