I still find it a stretch that the primal is sustained by the a doman myth. If that was the case Shinryu would be easy to deal with. We just would need to stop the Doman's thinking its Shinryu. After all, literally its only connection is a resemblance and honestly I could easily have seen them mistaking Midgardsormr or even Bahamut for Shinryu.

I am also not sure if I would expect Shinryu to be stronger, at least not yet. Bahamut had a long time acting as a conduit to get juiced up and damage wise both are calamity scale. Omega did apparently take down Bahamut but Bahamut had been weakened by a war of attrition at that point, starving him of sources of Aether, while using the voidsent possessed corpses of his own people against him.