Yes, it is indeed an original manga. However, j-novel does have one of the author's (Mikawa Ghost) novel works; My Friend's Little Sister Has It In for Me!

Giving this a bump cause Isekai-Alternative Romcom. I'd like this romcom licensed.

I spent ages trying to figure out the units in Youjo Senki; IIRC Crunchyroll used different units but the same numbers to the then fan translation of the novels, while checking the Japanese novels the units were almost always left out...