Garleans (spoilers)

[Vyrerus]

Garlean souls are still composed of aether, as shown by the Reaper job quests, and also by Zenos having the ability to manipulate aether freely due to the Resonance procedure.

They might be more resilient and more in tune with Dynamis, maybe. But there's nothing concrete as to if they can even perceive it, either.

I'd chock it more up to being that they didn't turn any Garleans to Blasphemies, because they already showcased their portion of the suffering pie this expansion.

As for why Thancred was the last to succumb in combat with Meteion? Well, she cut off our air, and Thancred had taught himself to hold his breath for 10 minutes or something like that, as explained in Stormblood when he manages to swim into the residential section of Ala Mhigo alongside us, who have Beastmen blessing for breathing underwater. That blessing doesn't extend to lack of air, whereas Thancred's training does.

Combine that with him being one of the Scions to experience the most suffering, and actually admit to have embraced it before through an implication from his speech in Thavnair at the start of EW. That's why he was the first to combat Meteion. He retained consciousness long enough to strike a blow at her. Short of having that kind of training, anyone else would have passed out from lack of air just as quickly as the WoL and the other Scions did, regardless of their affinity for Dynamis.