Building on this idea.

You have some partial results with your stud finder.

Knock around and see if you can hear where it is very solid. Combine those finding with the stud finder and magnet results. You'll find somewhere that you are 90% confident in.

Then, drill a tiny test hole and see if the drill is still getting resistance past 1/2" depth and comes out with some wood on it.

You can do this! I believe in you!