So, a little background- I've just come out as an atheist to my dad, a Christian pastor, who's convinced he can "fix" my thinking and is bombarding me with a number of flimsy arguments that I'm having trouble articulating a response to

Being articulate has nothing to do with the truth. If your dad isn't willing to explore where he's wrong, then you shouldn't be talking about your world views with him. If you can't establish your world view without him, then you're not ready to establish it at all.

I'd advise not worrying about "the big questions" so much as what kind of person you are in the relationships that mean the most to you. I suggest creating value in the world. What kind of person you are "in the small" is actually more complex and more rewarding to explore.