Imo Ymir isn't a threat on the level of Surtur and Odin, He isn't going to be the main problem here, Surtur and Odin is.
I have heard many times that people think that if Galactus is weak then he loses against Odin alone.
Here in this particular thread he is sated to a significant degree that imo is large enough to pull a win against Surtur and Odin, again I doesn't see Ymir as a great of great enough concern to Galactus to count him maybe I'm wrong...