I also notice that if I manually select placements (by selecting Facebook Feeds & mobile), it narrows my audience to under 1,000 which doesnt allow me move forward.
Advantage+ can be a good option for improving tests. It’s more useful for campaigns with many different ad sets, though. We will have a training on that soon.
About the audience size, change other things in your campaign. You may be restricting the audience with other targeting options such as interests, behaviors and demographics.