Thrawn is a Grand Admiral, but he is dissatisfied with the Empire's system and reality.
Unlike Moff, he is in a position to formulate military strategies only, so he cannot criticize them.
Tarkin is desperate to strengthen and maintain the centralized system, and has no intention of making it low-cost and efficient.
He is not very willing to give his opinion to the Emperor, and will not point out when the Emperor is wrong.
I think he is a typical yes-man.
Thrawn despises Tarkin for that.
Tarkin did not think carefully about the huge waste of costs and internal exhaustion in a centralized system. Thrawn saw through that.
Tarkin is the one who increased the risks of relying on the Emperor during the war.
I think Project Stardust was a failure.
There was little return considering the cost invested over many years. It was truly the product of an inefficient Empire. In the end, it was lost with just one proton torpedo. (Thrawn has no intention of stepping inside the Death Star)
From the perspective of military strategy, Tarkin is wrong. Tarkin values politics.
He was good at running a nation.
In the end, Tarkin dies spectacularly along with the Death Star.
The destruction of the Death Star was a loss for the Empire, and Thrawn regretted not being able to advise the Emperor to break away from the wartime regime. Unlike Moff, he is in a position to formulate military strategies only, so he cannot criticize.
There are many other yes-men in the Empire besides Tarkin. Unless the Emperor dies, it is impossible to change. To my dismay, the Empire also builds a second Death Star.
(The existence of the Death Star is convenient for maintaining the regime)
Thrawn survives in the Empire while hiding his own ideas.
A wise man does not make enemies. After his death, Tarkin's achievements are praised.