
The whole miller fam is already in the future in kiss me liar

Who are you talking about? Pete and Cecil are still kids.
Chase's sibings? No, at least not all. Grayson is still single when the last extra of the KML novel ends, because his story starts a few years after what happened at the end of the last extra (would be a spoiler, since not adapted to manhwa). The others found ther partners before Grayson, but I don't know the when, because Nathaniel and Bliss's story are still not released, yet. And there are no stories planned for his sisters.

Not even his webnovel is released so far.
The author delayed it 2 times, but it should come out after Grayson's webnovel is finished. So I don't think we'll see a webtoon in the near future.
Grayson's story, although was the first the author came up with, is chronologically the last one of the planned Miller novels, so all his siblings already found their partners.
Concerning Nathaniel, spoilers and things I read, nut I can't guarantee it's correct: partner's name is Chrissy Jean? He is most likely the prosecutor we saw in the newspaper article in chapter two of KMIYC. Beta who will be turned into an omega.
Some others wrote is he is promicouous (like Josh and Dane.....) and had a partner, but not sure about that. The author also released songs that she associates with Nathaniel.
This is the best