Those aren't things that would otherwise be crimes. He doesn't have immunity from procedure, he has immunity for crimes. He kill the justices, or kidnap them and lock them up in some undisclosed location. He has immunity in those cases. But expanding the court would require passing a law. Passing a law is not an action that the President takes, regardless of any presidential immunity. As for felons not being able to become presidents, any law congress passed to say that would be unconstitutional, because the constitution lays out the only requirements to become a US president. The constitution also limits the ways in which the constitution could be changed, and none of that is within the powers of a president. He could kill Trump, but he can't change the rules about who's allowed to be president.

He still believes that the system works. He thinks the checks and balances work. He believes that, regardless of the recent Supreme Court ruling, that he's not immune, so he won't commit crimes like that. The result might be that the final president of the Republic thought it was more important to follow tradition and live the values that he thought the president should hold, than to do what was necessary to prevent the Republic from becoming a dictatorship.