If the goal is simply to make social media unprofitable, you can just be really brutal and require that all users from a language region visiting a website (or using an App) must be delivered the exact same set of ads.
The fact that most advertisers would flock to promoting on smaller special websites/apps (equivalent to your gardening magazine), is exactly the side-effect we want. The shift in spending will hopefully lead to the current "massive social media platform" model will dying out, and boosting smaller independent platforms.
What’s the benefit of a smaller independent platform? They can be just as problematic…
I’m not sure if you’re just fantasizing about the “good old days” of phpbb or something.
Smaller platforms are worse at optimizing for engagement. It's one of those weird situations where it's a good idea precisely because it's economically inefficient.