Well, if you presented them with list of 100 partners each with 20 page of privacy policies and they accept it within 10 seconds it should be tricky to argue that user actually read it all.

You could, for example, require that user answers very specific questions regarding 10 randomly selected partners and how exactly they can use the data ("is partner x allowed to build very detailed profile of you and target you with political adverts that are designed to manipulate you?").

Is that not what the courts are for? I imagine that if a court had to enforce a requirement like this, knowledge would generally be the best kind of proof. If you know what the contract said (or even it's terms in general) that would be enough.

The reason this isn't done is because corporations legal departments love writing 10-100 page contracts that absolutely nobody is going to read.

Well for instance if the other party is pressuring you to sign it then it can't be assumed that you had adequate time to read it and understand it