I always felt Kirk's Nexus fantasy should have been him aboard the Enterprise, in the captain's chair, with the TOS crew. To exist in a fantasy where he's still in charge of a starship AND knows David is alive and well.
I realize that was probably not possible, since Nimoy and other cast members had made it clear they were either not going to show up for something that amounted to a short cameo or they felt they were done after The Undiscovered Country.
The mountain cabin and woman upstairs waiting for him to bring breakfast never felt right for the character.
I don't think the movies leads one to believe Kirk was obsessing about the "one who got away" or may have secretly desired or wanted a family. I don't get the feeling he came to regret the choice of a life in Starfleet. If anything, I think everything leads one to believe that Kirk is only really happy when he's allowed to be in his "first, best destiny."
I always wondered if there's anything to be taken from the way leadership and the captain role is usually written in Star Trek that only 2 of the Trek series show captains in committed relationships or marriages (i.e., DS9 and DSC with Michael, Saru, and Sisko)? Kirk, Picard, Archer, Janeway, etc., are shown to be "in love" with their roles and the purpose it gives them, to the point that they've sacrificed the idea of having significant others in their day-to-day life.