In my head, after the space whales jumped the Chimaera away, Thrawn shot Ezra in his dollar store knockoff head and used his tactical genius to kill the space whales, then returned to the Emperor and was assigned to the Unknown Regions to set up the shipyards and manufacturing that the First Order used to build their armies.
Oh and then when Sabine and Ahsoka went on their odyssey, he killed them too and had Ahsoka’s head stuffed and made into a trophy.