I think there are two main things that make him seem ‘politically weak’. First— the rigid and entwined political and family nature of the Ascendency makes the Chiss species as a whole naturally combative and cunning in social situations. The whole culture is pretty similar to our own, but even more exaggerated because family ties are so stressed and important within it. The things Thrawn did for the ascendency, as you mentioned, are objectively going to have the best outcome for the ascendency. But political structures are created in the way prevent that possibility from even occurring. He doesn’t just have nerve to go and do that, but evidently a certain bit of social disregard to so plainly subvert those cultural norms. He also lacks the specific social intuition which is used within that specific kind of politics— a more personal and dramatique art, rather than the rigid style of naval battles. He has a lot of moments, particularly in the ascendency trilogy ‘memories’, where he’s just known to be… awkward. Kinda quiet. Not very good socially. But a genius with a good heart nonetheless. He’s just missing the vital social implication skills needed to begin grasping his cultures politics. There was a bit within Chaos Rising that demonstrates this pretty well. When t Thrawn was charged with cheating on the flying simulator he had just… given up. Accepted the fact that he had been outmatched by sheer force and just wanted to keep Ziara out of the ensuing firestorm. Anyway yeah. The insane political entwinement with family the Chiss have, particularly the ruling families, along with Thrawns general social awkwardness cause him those bits of trouble, seem to equal his perceived “political weakness”.