If my advertising topic is now on the 9th page (hasn't been posted on since February), I can now make a new topic with the same items, plus some new ones, correct?

You're totally in the clear to make a new post, since you can make a new topic after your old topic hits the 3rd page

That's what I thought, but that annoying voice in the back of my head kept asking "Are you sure.....?"

Thank you

Just confirming that yes, once it's past the third page, you can remake.