Her weakness is going to be blocking her butthole so that the next time she tries to tunnel through the sand it all gets stuck inside her and makes her immobile and eventually starve to death. I don't know how Youta is going to abuse that weakness though but I believe in the author's imagination.
I'm thinking sea water or salt is her weakness. When mc and Lumi boat sank on their way to the desert city, there is probably that Lumi sword got some sea water or something back then and it became effective to Galia.
If she truly is a sand worm and this goes by Japanese video game logic then the weakness to sand worm monsters in video games is fire. But those games don’t have girls transforming into the sand worm so idk what to say.