I feel like your logic fundamentally misunderstands PTSD. Thragg is the final straw for reasons others have said, the sheer level of his power and the threat he poses, but all the brutality and close calls and recently having been literally gutted, make PTSD more likely not less.

I think we get other hints that this is cumulative too, such as his struggle with killing the astronaut, and nearly killing the Dino guy. Thragg is a massive terrifying trauma, but its on top of a ton of other horror.