Co-author here of the research. The most simple and effective and rapid solution would be to impose API neutrality. As explained in the report, it would just obliges API providers to give back the same API access to users than they give to their partners. For instance, why I get less data from Facebook if I ask my personal data, than if I create an app and ask maximum app permission (all OAuth scopes)? API neutrality already works. For instance, Open banking in UK and PSD2 in Europe apply API neutrality. Any 3rd party can access to a bank API if they are granted by the user to do so. After 2 years, for instance, up to 20% of the UK online banking population beneficiated from it as "Banking data Portability via APIS" . 20% is huge. If FAMGAs and all other big companies data was accessible via "neutral APIs" to users, data portability would be "a thing"

Also, the fact that you don't know what to do with you data dump in JSON is a blocker. With APIs, integrations by 3rd parties are simpler and more user oriented.

Last point, with API neutrality, no need of maximizing "interoperablity" (even is is always useful and makes things simpler, we have seen that with DataTransferProject it does not work really as companies don't work with the same data model) Developers will do the matching work between the original app and the destination app, no worries, when incentive is here, middleware glue will come. The problem these days is that the source of data is useless, has no value, so no incentive. You can look at this study with GDPR Facebook data value for developers https://www.law.nyu.edu/centers/engelberg/pubs/2019-11-06-Da... The main question is : Why a Facebook GDPR Data dump/takeout has no value for developers where Facebook API has value for millions of applications developers and businesses? With API neutrality it will have maximum value for users (as it has already value for partners) and minimizing fatigue to implement portability (an API is lot more developer friendly than a JSON dump that you receive in 30 days via email and that the user need to upload somewhere)