That still doesn't solve all the political dealings that happen but it would be a good strong start to cleaning up a lot.
Follow this up with certification by a main industry body and legal foundation for how companies in the sector should operate with data and we can finally make progress. This is no different than how finance companies or health companies go through audits and such with both process and data handling.
While digital advertising doesn't need to be as onerous and involved as that, the same workflow would help along with clear rules for collecting, analyzing, acting on, purging and securing data as part of consumer wishes and best practices. The next step would be to make sure transacting with a non-certified company carries risks and perhaps fines so that media buyers actually work with good vendors based on merit instead of inside deals.
There should also be technical certifications so that companies can actually prove they know what they're doing and dont just cram megabytes of crap on a page to serve a single ad. It's just too easy to whip up some crappy ad server software these days which ends up bloating sites and ruining trust and user experience.
Unfortunately there are issues with regional laws applying to a global business and the large amount of shady networks headquartered internationally but it's a good first step.