The first problem is that God is not conceivable by the mind of a creature. If you want to start out saying that it is possible that God is not conceivable by the mind of a creature that’d be fine, but usually theological speculators are completely ignoring this possibility. The tendency has been to think of God as being like a “big man,” not an Entity with radically different and greater properties, so much so any created mind could not “see” it.
To give a small example, I’d submit humans are not able to conceive of what an octopus is doing, wielding eight arms simultaneously. Though the neurons can be traced, our experience is having four limbs that are more or less coordinated, most of the time. So we can’t identify with what the octopus is experiencing; we can’t properly conceive of what it is doing, to wield its eight arms. God is doing everything all at once; it is inconceivable.
From this it becomes obvious the argument is flawed starting at 1, that if God is the greatest being, of course it is not possible that there would be another greater, putting the question of whether humans could even understand what that might mean aside for the moment. Again using a practical example, suppose there are a number of sharks in a tank. Your proposition seems to succeed until it gets to the biggest shark, where it clearly fails.
