It is essentially an attribute of his attribute(will) making it non essential. So since God an his essence are 1 as in god is omnipotent and omnipotence is god ( divine simplicity), would it not be accurate to say mercy is God and God is Mercy because of them not being essential attributes?

We can metaphorically say God is mercy and mercy is God, but if we mean it literally then it’s not accurate. We can accurately say God is merciful. But when it comes to knowledge and power, we can say God is power, or God is knowledge.