The issue with your quest is that lots of advertisers, especially the big ones, offload the targeting to Facebook internal ad algorithm. Meaning they keep spending money and feeding the algo with conversion data until it "learns" how to target better on FB's end. Short of an expensive insider service, there is no way to spy on exact interests but you can get basic demographics data about targeting and location. Coming up with targeting is not that hard as well, what is much harder is coming up with an engaging ad creative that leads to high ROAS. Data on what runs long and gets lots of engagement is part of several Facebook ad spy tools and services and about as close as you can get to "bulk spying on a budget".