A thought here.
Professor mushroom head suggest the Kaiju’s natural prey might be something like the metal shelled gastropods, means animals that use metal in its structure. This is different from actually eating metal.
If we assume here that the Kaiju’s metal processing ability is unrelated to getting energy and nutrition (the ability is strictly to access the flesh, with side effect of evolving defensive traits such as generating heat and spitting molten metal), it would go along with how Reina keep saying it is hungry.
Even with symbiosis with metal eating microorganism, it would still burn through a lot of energy to move. Yet, everything it ate like the tower or cars are like eating empty shells, wasted effort that only serve to make it feel even more hungry.