Sounds to me that you are the one choosing fantasy. Just because LBJ was a sleezebag politician (most are) does not mean he conspired to kill JFK.
Read the link I gave you. Johnson knew nothing about the Kennedys using the CIA and even signing up the Mafia to kill Castro and how dirty Bobby Kennedy's hands were in that effort.
Johnson knew his days were numbered in 1968. There was a strong family history of heart disease and he had already suffered at least one and I think more than one heart attack. Within a year or two he was essentially a cardiac cripple and within a year or two more he was dead.
He also hated RFK more than anything and when McCarthy did well in New Hampshire, I think LBJ saw the writing on the wall. I think he did the political calculations and saw RFK entering the primary (which he did) but of course, did not know that he (RFK) wouldn’t live to see the convention. If you look at politics historically you’ll see that very often when a politician decides to retire, it’s often (not always but often) due to a calculation that they’re going to be tossed out anyway.
I think LBJ’s decision was partly health related, partly based on politics, and partly based on the toll Vietnam had taken on him.