Do that only if it looks like the campground will start to fill up. If you get there and the place is dead, don't bother spending the $$ for the additional site on Thursday night. Also, you probably don't need to pay for the entire weekend at once. I think you can pay as you go.

There is a possibility that after we get there, if our campground "neighbors" are rowdy and inconsiderate, we may go for the "backcountry" option.

I appreciate your willingness to get there early.