@Solipsist Empress of another world might be to more to your taste, then. You can find it in Kissmanga.
As for this… I think the fact that the past five years have been mostly their growing up plays a role. Even if she had such prescience based on the novel to be a strategist, there wasn't much to strategise about, since they have been kids so far. Other than escaping the castle to train with the mercenaries, I doubt Cesar was up to much, and neither was the Empress. It was in Canaria's interest to not prevent Cesar being sent to war (probably the only scheme of the Empress' in this entire period) because she believes it will backfire on her anyway. So as she requested in chapter 4, living the good life is the only thing she could aspire to in that period, and she achieved that by getting the Empress in her side. All she had to do was feed her inconsequential reports of Cesar's activities, which must have amounted to little more than a boy's antics. I suppose she could have traded information like the identities of the mercenaries, but the Empress would be very suspicious if she knew that, and if she asked Cesar he would the one suspecting her instead.
In her view, Cesar must get the upper hand because otherwise, as she said, Noah will be appointed heir and she will be stuck with Cesar (which is a bad thing in her mind because she thinks he doesn't feel anything for her or that he won't deviate from the plot of the novel); or Cesar will die and she will be sent to the monastery.
That's why she tweaked very little (in her mind) in the plot so far: she thinks all she has to change is her relationship with Cesar, so he will reach a position where he can decide to divorce but not behead her. (My guess is that like in post-Reformation England, divorces were something only available to the powerful.)
What I find interesting in this story is that Canaria favours Cesar for purely pragmatic reasons, and doesn't even seem to be particularly antagonistic towards the Empress. She wants to be in everyone's graces, and who can blame her?
In other words: she didn't spend the time scheming because there wasn't much to scheme about.
But yeah, she doesn't seem to take into account the knock-on effects of her actions to change the plot. However, isn't it a good thing that she intervenes so little, if we think about it? If she tweaked too much with the story, she would lose her foresight because events would resemble the novel even less. And then she would have nothing but her wits to go by, in an environment she's much less familiar with than her opponents. If she wants to tread on the safe ground of knowing the plot, minimal intervention is actually the best course of action. I mean, just being kind to Cesar and sensible would have sufficed to achieve her goal. She only went the extra mile with the Empress because she wanted peace of mind.
In a sense, that's more realistic than heroines who become grand strategists on the spot, I guess.