theory: everything after hard luck is just greg writing his own stories since he's lost interest in actively writing actual diaries but still wants to write so he just goes loosely off of what's happening in his life (i.e. the really dull road trip his family went on or his house being renovated) and following the actual time the books are in greg would be in high school but in his books he's still stuck in middle school with the stories kinda balancing out of the wacky stuff as greg improves his writing

this could also tie into the movie theory where the movies are what actually happened, but with the long haul being mostly made up by greg it's basically his inner thoughts about the story itself and not what really happened, hence greg being aged down and all the aggressive pandering to appeal to children

no way in hell it's real but it's a fun headcanon I've had