I agree with the OP, there’s something appropriately and deliciously dark about the piano version. The one in Crossroads is amazing, one of my favorite songs on the soundtrack, but you first hear it and it’s static and twang and weirdness, only slowly building up to the amazingness that is the FF theme.
In this last episode, though, the song starts off as a hesitant, simple piano melody, full of cathartic emotion from Kara’s dealing with her past…and then, wham, you’re suddenly caught on this amazing ride to the FF theme and going “Oh my god!” Even if you were expecting it (which I at least was), it still hits you hard. The whole episode was building up to it, but you didn’t really know how until it actually happened, unlike in Crossroads where you know something off about that song.