Earlier quoted context omitted.
Activists are already well aware of the need for proper opsec. Leave your phone at home, use a burner phone (and don't reuse it!), don't call any personal phone numbers,, don't use your normal accounts, don't blab on social media, and so on and so forth. Basic stuff, but sometimes people do get careless.
There should be a service where you can buy the aggregated information that advertising exchanges have on you, as well as any other shared details.
Can someone else aside fom the data subject file a GDPR request to companies?
If so it would be easy to just request reports from companies and compile them to some friendlier format.