Pr.2 begs the question, as you are assuming the conclusion of the premise, e.g., it's true because it's true.
You are making a positive claim about reality, i.e., an assertion that could be true or false: that nothing exists externally to the universe.
You first need to prove your claim, but you cannot prove it a posteriori, so you need to prove it a priori.
Isn't "external" a spatial concept? Doesn't "the universe" contain spacetime?
Why not?
1. Whatever lacks anything external to it does not have a cause.
2. Nothing is external to the universe - it is everything that is the case.
3. Ergo, the universe does not have a cause.