In the situation you described, I would go with more defense for him. He has low crit chance anyway, and the less he crit the more he exposes, and that is better: more debuffs means more ability block landing as well. Plus the expose will make up for his low damage.
That said, you will have A LOT OF FUN with Thrawn lead. I was suspicious of his zeta but once I used him in the lead, specially against debuff heavy rebels, you will see your team fly, clean, heal like never before. It is such a small thing but between all debuffs you get + the buffs your team do + that 2% is for each time you get/lose the effect (both positive and negative) you will regen a lot of protection. And your team will get extra protection from thrawn lead so it is the most effective Empire leadership for competitive arena.
For the empire!
PS: You can see some combos in action in my channel: https://www.youtube.com/channel/UCnvO5eJ9bXMo017gNnUuGBg - lots of Empire combos and arena trials :smile:
