The story begins when the FL and ML are children. The FL is the daughter of a powerful noble family that serves as the biggest shield against demonic invasion from the west. The ML is the son of that family's most skilled knight, and is on path to becoming the FL's future guard. The ML has a very vague recollection of a past life where there were skyscrapers and more advanced technology, and an excellent recall of a story set ten years from their current time (and told from the perspective of a remarkably ignorant OGFL).

For the first dozen or so chapters, the kids are hiding out in a cabin, waiting for a demonic attack to subside. But things happen, they leave the cabin around chapter 17, and they have to fight for their survival. There's a four year time-skip at chapter 41 that takes them to their teenage years, where the bulk of the story then happens (up to where I've read, at least).

The FL is a clever tactician, both in warfare and in politics. And the ML is always striving to push the limits of his abilities as a sword master, becoming a truly fearsome opponent.

when they sleep together for the first time, the author didn't shy away from writing the actual smut (chapters 123-126).

I strongly suspect that the ML has always been exactly who his character is. I don't think this is a case of a different personality taking over a character. Rather than an isekai, it might be more of a regression after his soul took a bit of a break elsewhere. But it also isn't quite a regression story either, since what he knows isn't from his life, but from the narrative of the "OGFL, " who had been astoundingly ignorant of any of the things actually going on in this world.