Pssshhhh, I don't like Sci-Fi but if it's really good, ill read it. I'll refer to MC as he, but the gender of MC doesn't matter. Looking for a powerful MC (could be OP, could just be strong). MC must actually know where his power lies in the spectrum. He must not be dumb enough to think "Oh no, I'm so weak" after defeating a strong opponent. These isekai OP MC are truly getting on my nerves. SAO's Kirito gets so much hate, but at least when he sees an opponent he knows he can beat, he will be like "I will crush you" and not the "What do I do?" like the current MC of isekais. MC knowing when to retreat if needed. This is one important point. I don't need the opponent to thrash the MC, then talk about his sad backstory as MC slowly gets up saying "I understand, but I can let you do this" and proceeds to win a hopeless fight. As long as he can escape, he must try to instead of talking BS with the enemy. MC should truly do what he feels like doing, as long as he is strong enough. I really hate the "A Demon-King like me will not help those hopeless fools" but proceeds to do that anyway. I would really like an MC who is, "Pah, Im the Demon-King, Ill help whoever I want to and do whatever i feel like". If MC is not OP, and he should think of potential risk when he enters a fight, and prepare accordingly. Unwanted harem novels are a complete no-go. If it's a single relationship, then MC should know who he likes, and not wander around and then at the last 'choose' one from many. The single relationships would remain single. If it's a harem, then he better not even think about choosing only one in the end. Otome-like reverse harem and a complete no-go. They are trash beyond repair, no amount of good plot will ever be good enough to make up for an Otome-harem. Also if MC is Female, she mustn't have a crush on a prince charming. Read one of those, and it was a complete turn-off.
Lord of the Mysteries Dungeon Maker Otherworldly Evil Monarch I think these fulfill your requirements but you should check out their synopsis first before reading them.
Virtual World: Peerless White Emperor, Its a CN web novel. (I know you want JP or KR but I really love this novel) Its male MC, and it really connects with doing whatever the f*** he wants. The story is really funny as well. But the plot makes no sense, and I feel as though the author just adds stuff because he think it would be fun. And I'm not sure if I misunderstood, but I think you want a smart MC that knows what fights he can take and what fights he can't, but he doesn't necessarily have to be a genius. Because this MC is an idiot, but its to the point where I don't know whether its acting or he is just that dumb. BUT he does know what fights he can and can't take. Anyway if you can figure out what the f*** is going on in then you might like it.
Hehe, I just put up JP and KR because after reading so many wuxia/xianxia I'm getting tired of "get-supreme-treasure-and-become-Mt.Tai", but I will read good ones no matter the lang. And this one looks really interesting. From what you said, and the fact that a CN novel has nearly a thousand chapters but still managed to maintain a 4.4* rating, I'm extremely interested in it. I agree, I have read it and it's great. Although I don't like this kind of MC, because most of them become boring after the shock factor of MC being "evil" wears off, this was a good read all the way.