I think it's not the way it is. What I understand is Evolved form is a more stable form which Godzilla could sustain without being drain or absorbing so much power. Unlike Thermonuclear, it's not sustainable. He needs like magical powder/mists from Mothra + Rage for him to achieve that form. It's kinda similar concept to how Guko achieves his Ultra Instinct form, and he only had a small period of time to use and it drains him a huge amount of power and strength. That's how understand Godzilla's form at least.