Non-existence

[Jon Paul]

My bolding.
And within that mind exists the thoughts and feelings of the possessor, as well as the experiences with the billions of other would-be ontologically independent minds and the rest of the stuff that comes with it.

That is not a necessary premise for solipsism, no. All it states is that your conscious mind exists, and it's conscious experience. It doesn't state that anything that falls outside your actual conscious experience exists even in your mind, because to state that would be unverifiable. And since it doesn't state that your sense-data exists independently of your conscious experience, the sense data itself, even if it was much larger than it is, will always be less complex than an actually existing reality, since sense data contains and records less than actually exists according to realism.

A mind capable of inventing and maintaining this is far more complex as an entity than an objective universe full of beings.

Perhaps so. But that is not what solipsism proposes. That would be proposing more than solipsism.

the process by which it came into existence is surely a mystery.

But that falls outside what solipsism affirms anything about. Solipsism says nothing about ontogenesis, only ontology. It doesn't affirm that there is any way to know anything about ontogenesis.

Of course it's absurd and there's no way anyone (or at least, any sane person) can accept solipsism. But that it is incovenient and absurd and morally and epistemically unacceptable is not a rational reason for rejecting it.

In the same way no god is less complex than an all-powerful, super-complex god.

Not according to the doctrine of divine simplicity, the orthodox Christian understanding of God, which explicitly states that God is the simplest possible being, and that all his attributes are identical to his being, and that the universe is more complex than God.