For much of this year up until Ahsoka and really ever since I read the first Canon Thrawn book in the spring of 2021 I was of the opinion that Book Thrawn and Rebels Thrawn were just two separate entities, and were essentially two completely different representations of the same character, and I much preferred book Thrawn. To this day, even now, I still prefer book Thrawn in terms of his characterization. But my opinion on the matter has somewhat changed, to where I've come to a place of interpersonal rationality where I can understand the ways in which Book Thrawn and Rebels/LA Thrawn are connected.
Thrawn initially joined the Empire because he wanted to see if they were worthy of becoming a valuable asset/ally to his people, the Chiss. Essentially, he joined them as a means to help his people. But through studying the Imperial hierarchy and coming to understand the true might of its military strength, Thrawn came to appreciate, and even prefer in some ways, the structure of the Empire - the structure of a fascist government. This is an established point in the first Canon Thrawn novels, in a conversation at the very end, where Thrawn all but says in words that he thinks the empire is a good system, but it is/was a currently flawed system at the time because it was being run by a man like palpatine.
In the Ascendancy trilogy, specifically the 3rd book, it's also emphasized at the very end, right before Thrawn's exhile, that Thrawn is willing to go to great lengths - by all extensive purposes, willing to do practically anything if it means keeping his people safe. Thrawn during the empire (in both rebels and the books) believes in the structure of the empire, and he believes in the very real and great threat to the ascendancy in the unknown regions. It can also be interpreted that part of the reason why Thrawn sees the potential of the empire is that he comes from a society mired with political infighting, and in the ascendancy books, he almost witnesses his entire civilization crumbling because of glory hungry individuals who sought personal power over the well-being of the strucutral whole of the chiss. The empire, unlike the chiss, is run by one individuals Palpatine's vision is final, and any dissenters are dealt with. And to Thrawn, this has provided stability and a strong central government that isn't easily susceptible to political infighting, like how the chiss almost were during his time there, because of there being only one figure in charge. (problem is, Thrawn is wrong about this - but Thrawn being wrong isn't a bad writing problem. It makes his character more complex, nuanced, and even tragic, in my opinion). Another thing about the empire that differs from the chiss ascendancy is that the empire is willing to go to great lengths to achieve victory, such as being the first one to instigate, unlike the ascendancy's "pacifism" rule (where they are never the first one to strike.) Thrawn taking that initiative and going against ascendancy protocol arguably was the reason why the chiss won the day in the end, and yet he was punished via "exhile" for this. The empire doesn't have this problem: Thrawn is encouraged, really, to do whatever it takes, as long as he just doesn't commit direct treason to the emperor. Having watched the show now, and having considered the Thrawn books from differing angles from that which I used to look at them, I wholeheartedly believe that Thrawn believes that the empire is the only solution to establishing order and peace within the galaxy. I think of it like this: Thrawn serves the Chiss Ascedancy, above else. He believes there are very real and grave threats out there in the galaxy that the Chiss Ascendancy, alone, cannot handle. He serves an empire, and by all extended purposes, he is loyal to this empire as one of their last grand admirals. But the reason he serves this empire, while still being above all else loyal to his people, is that he believes the empire is the key to helping the chiss ascendancy and that without the empire, his people, and the galaxy at large, are doomed.
