I think Ascendancy is a good read for anyone that's watched Rebels, because it adds a whole new angle to Thrawn. And I'm going to preface this by saying what he does is still pretty messed up, and I'm sure many of his friends in the Ascendancy would kick his butt if they knew half of what he'd done, so this is in no way me trying to brush it under the rug. Also, spoilers ahead for Ascendancy, and the end of Rebels.
But reading Ascendancy and learning that he joined the empire with the intent to find help against the Grysks makes him so much more interesting.
Because suddenly he's not just some selfish, imperial tyrant who takes art from worlds he destroys in the name of cultural preservation. He's not taking over these worlds because he enjoys it or because he wants to steal their cultures and art, infact I'd venture to say that if he wasn't the reason their cultures were going extinct it would be a somewhat nobel venture (like when Uingali foar Marocsaa gave him his ring in hopes of preserving a piece of the Paccosh).
Suddenly he's Mitth'raw'nurodo, senior captain of the Chiss Expansionary Defense Fleet, so devoted to his people he'd continously risk his career for their safety. A man who's name literally ends in "guardian" and who was scouted by the patriarch of a family who's emblem represents one of the greatest (if not greatest) sacrifice in the history of the Ascendancy. A captain that even though he pulled insane stunts, his crew trusted him to come out on top.
The man Thoorkai, Ba'kaif, and Lamiov thought could rival the tragic Patriarch in terms of greatness and ability. The man Thrass saw as his brother and entrusted his greatest secret to. The man Ar'alani, Samakro, Ziiara, Ba'kif, and Lamiov were willing to stand with in the face of an insane plan that even if successful would still have terrible repercussions because without their trust there wouldn't have been any more volunteers.
Thrawn that Ar'alani risked her record on after not even knowing him a day.
Thrawn that Thalias chased after because he showed her she still had purpose.
Thrawn that Samakro, a complete stickler for regulations, was willing to risk his career to follow.
We see him engineer plans that will result in as little backlash for his crew as possible, and encourage people around him on multiple occasions.
His actions in Rebels are just him trying to earn favor with the empire for his people, because he's a loyal soldier and his loyalty will always be to the Ascendancy. Even though they exiled him in an act of petty aggression, used him as a scapegoat for their own short comings, he stuck with it and made the best of it for their sake.
So even of you don't read Thrawn, I recommend you pick up Ascendancy because it is still very much the same character, we even see Grand Admiral Thrawn poke through a few times, but he's also more than what Rebels could show us. He was the villain in Rebels, and an amazing one at that, but Ascendancy makes you feel for him. When he's dragged away at the end, you feel for Ezra because we've followed him throughout the series and we know what he's left behind, but through the lens of Ascendancy I've started to notice that he and Ezra are far more alike than Ezra (and maybe even Thrawn) realizes.
Infact, I would say Thrawn and Kallus are fairly alike too. Both Thrawn and Kallus are just following orders to survive, and Thrawn and Ezra are both fighting for the future of their people. Though Thrawn's methods are sometimes more akin to Saw Gerera's methods, didn't Ezra give into the sith holocron in hopes of saving his people? Is that not similar to Thrawn pledging allegiance to the empire?
On another Thrawn and Ezra note, I feel like their relationship is similar to Negan and Carl from the walking dead.
(Is this even completely coherent? I wrote it over a week while finishing Ascendancy)