I think the movie does its darnedest to establish that Khan has no plan - that the once brilliant tactician is now a raving lunatic who sacrifices everything to his quest of hurting Kirk.
Exactly. Khan had an entire starship
at his disposal and could have gone anywhere in the galaxy, but what did he do? He picked a fight with an old nemesis to settle an eighteen-year-old score that was largely a fiction brought on by his mental and emotional deterioration-- a fight he ended up losing because he never considered the possibility that despite his advanced intelligence, Kirk was simply better
than he was when it came to commanding a starship.
I could spend hours talking about how I believe much of Kirk's command style as depicted in the TOS films was merely leadership by arrogance rather than tactical brilliance, but that doesn't mean he didn't have moments when he managed to put his own ego aside for the good of ship and crew.