I assume your argument here is based on Plantinga's work. If so, a key thing you are ignoring is the primary axiom on which the argument relies, 'Axiom S5'. The modal operator states something along the lines of:
If it is possible that something is necessarily true, it must (logically) therefore be necessarily true in at least one possible world.
If something is necessarily true in at least one possible world, it (logically) must be necessarily true in all possible worlds.
To demonstrate this, you could use a basic math sum as an example. It is possible that 2 + 2 = 4 is necessarily true, which means that in at least one possible world, 2 + 2 = 4 is necessarily true. However, if 2 + 2 = 4 is necessarily true in at least one possible world, it must also be necessary in all possible worlds.
To counter this axiom, with this example, you would have to argue why a world in which 2 + 2 does not equal 4 is a logically tenable possibility. This is fruitless though, because at this point you're arguing against a predefined set of terms, that are almost universally uncontroversial. The same can be said of Plantingas ontological argument and his concept of maximal greatness. The only way you might squeeze out of Axiom S5 as having these implications is if you, like James Garson, argue that necessary and possibility don't mean what Plantinga intends.
When the effect of this axiom is taken into account, the modal ontological argument can no longer be said to be begging the question, or circular reasoning. It simply demonstrates that if it is possible that the existence of a god-like being is necessarily true in one possible world, it's existence is simply necessary (in all possible worlds, of which ours is one).
Depending on your conclusions about the internal consistency of the definition of maximal greatness, this argument succeeds in proving that a maximally great being is either necessary (and thus god exists), or impossible (and thus god does not exist).
The problem with unicorns and pizza, and other silly replacements of maximal greatness is that there is nothing intrinsically "great" about any of them, especially in the sense that Plantinga means.
I'd strongly encourage you too study his argument more closely.