Actually your argument is circular. You are trying to prove that there is no causal agent outside of the universe by assuming that there is nothing outside of the universe - your first premise.
Of course, if you assume there is nothing outside of the universe, then there can be no cause outside of it.
You are assuming in the first premise what you are attempting to prove as the conclusion of your argument. The definition of a circular argument.
However, you are still stuck with the implication that absolute nothingness can cause the universe which you by definition claim is ‘absolute everythingness.’
So the takeaway from your argument is the opposite of what from the time of Parmenides was understood as a philosophical truism:
Ex nihilo nihil fit (From nothing comes nothing.)
Your use of quantum mechanics to support your claim that something can come from nothing – I.e., “Some events in quantum mechanics are uncaused, like beta decay or pion decay.” – confuses unknown cause with no cause. Merely because a cause hasn’t been (or, even, cannot be by our current knowledge of physics) determined does not demonstrate there is no cause or that ‘nothing’ is, in any valid sense, a cause.
If you want to assume that nothing causes something or anything, then why don’t we witness all kinds of entities and events that occur from nothing (I.e., no cause) all the time, around us? Why don’t we simply give up on science and laws of physics altogether and just assume anything can happen for no particular reason at any time because nothing can be causally efficacious?