We've used Insites Cookie Consent and OneTrust as part of GDPR-aware cookie management solutions.
However, you won't be able to implement such a solution without a Marketo-aware developer. These JS helper libraries are a critical piece, especially on the UI side, but they do not contain the logic to enable and disable all tracking cookies, and they have no awareness of how Marketo's Munchkin cookie works. (You also need to be attentive to email click tracking, which does not use cookies but is part of activity tracking.)
There's no shrink-wrapped app that's going to do this job for you... and you're already behind schedule!
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