Eunuch? I'm the Soldier Who Rules the Palace

An elite modern soldier wakes up in the past inside the imperial palace, in the body of a menial servant who is being held for sentencing and expects to be executed within days. He talks his way out of it. To survive after that he hides what he can actually do and passes himself off as a junior eunuch, moving through the inner court where nobody looks twice at him. He also finds he has access to a system that works on relationships: as he gets closer to the consorts and princesses of the palace and their regard for him grows, entries light up and he can trade them for a stronger body, presence that makes officials give way, and martial arts of a level nobody in this era has. The palace is full of people setting traps for each other and he uses those rewards to get out of one after another, building a base of his own inside the harem. The higher he climbs, the more people start looking at where he came from, and the secret that he does not belong to this century gets harder to keep.