And I still don't see the problem with this, unless you are strictly speaking from a point of "what would be more interesting to watch". But from a plausible story development pov, Sylphie is a-ok.

Let's look at Eris. What would have happened to her without Rudy? Well, first of all, she'd be dead, lol. But let's assume she'd have survived the teleportation event, then what? Or let's assume there never would have been that event. She would have stayed at home, have trained a little sword fighting so that her father and uncle get some freetime, and ultimately she would have married some random noble guy. She would have ended up as "WORSE" than Sylphie, because at least Sylphie chose family life on her own. Eris would have been forced to become a noble lady, giving children to her husband, while wasting away her time with mundane bs, not even doing cooking and such, because her servants would do it for her.

So please, the whole "Eris had so much more ambition than Sylphie" argument is simply the consequence of things that were forced on her. Had things proceeded naturally, she'd have ended up in a worse life than Sylphie and she wouldn't have fought that life. See her ever-pissed off mom, she would have become exactly that.