Everything's dangerous. And she's not wrong that it's one of the more dangerous things to do but - maybe not if it's impossible to sneak up on her. (...So maybe the trick for hiding thoughts is simple and the kind of thing you can just tell is true once you've run into it, so that actually lots of people will be able to sneak up on her soon. Maybe that's why she still thinks it's dangerous. That or it's just that she needs to sleep sometimes.)
Not knowing what to do with a country is - really constrained ambition. Teru doesn't know anyone like that. Are serfs like that? Maybe? Ruling a country would mean being able to demand a house be built exactly to her specifications. It would mean being able to order five people to take babysitting shifts. Teru would want it because... if there's not enough food for everyone, the Star-of-Stars isn't one of the ones who starves. The Star-of-Stars never has to worry about kissing up to people, and conversely, everyone respects him. No one is going to order the Star-of-Stars killed - or, well, they might, that's how assassination works, but he's not definitely hostage to his brother's good behavior every time there's a war. And she could change all the laws she doesn't like and offer her family nice things. But for any goal she could possibly have, more power would make it easier to achieve.
Should she be entertaining this train of thought? Absolutely not. But she's running low on willpower, and anyway, if her safety is based on Beka not realizing obvious things then she isn't safe and she might as well find out now.