Simply make it illegal to base the choice of what ad to show on any data derived from the person accessing the content. The same content accessed by different people from different locations should have the same ad probability distribution. You can still do old-school targeting by associating static content with certain types of ad a priori, as long as the shown content is independent of the user and not generated from any user data.

I'd happily support that but the harms of advertising go beyond the problems of surveillance capitalism so heavily restricting ads seems like a good idea on its own.

I feel like all the “targeted audience” stuff is used more to sell ad space and get its metrics rather than actually “targeting” ads.

That's only part of the problem.