I would go to the manufacturer of the FDR and CVR and take some experts with, which do participate in the examination and restoration of the content. Should be plenty of experts around in the different national agencies.
Then get a copy of the digital content and take it to a safe place. From that on any tempering with the evidence is impossible and can always be proven. So the critical path is until you can read out the digital content. You can be sure that Boeing, GE, Safran and all the current Max users have a high interest in a timely and honest investigation. Is enough money at stake here.