Thanks for sharing. You did everything the right way. 100 isn't bad for a first conference with a seed of 80-100 regulars from a meetup who know your style. The percent of each group would be insightful. Some thoughts.

The people going to the weekly free meetings might not be the audience for the conference. Group meets months (weekly?) for free those friendships get renewed often and don't require an expensive meetup.

The FB ads are not working, it might be the ad content. You mentioned a/b testing but it sounds like a low % of signups. It might be the wrong medium. It's probably targeting poorly. A sponsorship deal with popular twitter/ig/podcasts personalities would have been more targeted and introduce you better.

The other thought is perhaps people don't want your type of local conference for the price you are offering. I might pay $500 to go to a conference in Europe (plus airfare+hotel) because it's an experience but I would never pay 500 to go to a conference in my city because it lacks that global experience that justifies the cost. The price drop to 150 makes it seem less valuable.