All the dead become an Osiris, but do not become Osiris. The Coffin Texts, including the Book of Two Ways, and then the later Book of the Dead are guides for commoners to get to Osiris and to be judged, and how to respond to the questions in order to pass judgement. The king does not stand in front of Osiris to be judged, he gets a free pass. I'm sure this much will be agreed upon. So, what becomes of the king after his death. he of course becomes an Osiris, but, after being greeted into the Duat, he jumps onto the Nightboat and travels with the also "dead" Ra, known only as "Flesh" while he is in the Duat to signify his weakend position. Ra does not need a roadmap to traverse the night hours, but the king does as when he has died it is his first journey through the Duat. As Ra (Flesh) is the central character in the hours, it appears as if it is all to do with him and not the king, who hardly gets a mention, but the king is there all the time, even if not depicted or named, because, as stated, this is a roadmap for him, not Ra.
It is only with Ra that the king reaches Osiris in the "Hidden Chamber" at the 6th Hour, and it is with Ra that he, and by extension the rest of the dead, are resurrected, or rather, have our "spark" re-ignited, rejuvenated or whatever. The king then continues his journey on the Nightboat with Ra until his rebirth at dawn at the end of the 12th Hour, or equivalent in other roadmap books for a king, and continues his journey with Ra in the Dayboat, and the cycle continues for eternity. So the dead king, while becoming an Osiris, which I view as perhaps more of a euphemism to show they are dead without being explicet, spends eternity with Ra, and every night becomes resurrected with Ra at the "union" with Osiris. And what a suitable post for Easter.