Here is a French guide on anonymization technics that are considered as acceptable, in French and not sure the original document is translatable so maybe it’s more for you @Augustin (https://www.cnil.fr/fr/le-g29-publie-un-avis-sur-les-techniques-danonymisation-0). Also read Legal conformity: what we need to do to understand the broader regulation that will influence some of the decisions we take on the deletion.