It was a webtoon. Most probably Korean.

The number "1000" was emphasized as the number of loops the female MC had lived through before the start of the serialization.

The character in the story is supposed to hate the male lead? I think? And they're betrothed in a way?

At the 1000th or later loop, she makes sharp changes up her approach, and one of them is getting closer to the male lead. But she doesn't mean to, and doesn't try to honestly seduce him. It happens naturally when she tries apathetically to joke around with him about a grim event?

The male lead suddenly responds favorably, seeing her in a more respectable or at least important light. He's still not exactly kind or infatuated with her though, so he still looks pissed off somehow.

There were recaps for it on YouTube.

That's all I remember 😀