Lemma 1: you want to protect your users privacy, and are also beholden to regulation enforcing that commitment (GDPR).
Lemma 2: you are obliged by other regulation to offer equal access to user data to third parties, so others can build equivalent functionality (DMA).
Lemma 3: malicious third parties will absolutely try to abuse the access and trick the user into sharing their data by all means possible. You will be held responsible in court of public opinion at minimum and legally at maximum if/when a malicious third party abuses said access.
This is a hard, possibly technically unsolvable problem no matter how much money you might have, because the root issue is not technical, it's the fact that you legally have to give third parties access and no way to control what they do with it - and as others have mentioned in the threads, it's exacerbated by the fact that the regulation doesn't say "this is okay and this is not", it is vague and judges things "by outcome", so you may spend all the time in the world implementing a solution you think will work, and then get hit by fines/lawsuits because the implementation is judged as not sufficient after the fact.