I’ve thought about it a little more… Turns out a tracking pixel is only used for retargeting, so I won’t be using one (At least not now).
So the plan would be to build different campaign links for each ad. That will at least show me how each ad (topic) did as far as new users, time on site, #pages viewed etc.
Is there a page that I can have users “land” after they signup? Then I could set this page as a goal in GA.