An independent NGO or watchdog kind of regulatory body staffed by psychology and technology experts with the legislative teeth to issue cease notices with meaningful fines to back them up (take the GDPR penalties as inspiration) should a media platform implement a harmful user experience.

It would be important that the organisation is protected from lobbying interests and direct control from the elected politicians. Though obviously its existence would hinge on the corresponding legislation being persisted.

The result will be a couple of big fines and then everyone else will play ball