Earlier quoted context omitted.
Merely setting a delete flag is not compliant with the GDPR, that's why a cascading delete is necessary. Any programmer worth their salt knows mass random deletes and updates are extremely inefficient.
Wouldn't it be possible to just delete the 'idetifiabel' parts in the database in order to be GDPR compliant? If you for instance save all the user data like user preferences under a random userId, and then delete the personal data (such as email address, name etc.) associated with the userId I would expect this to be GDPR complaint without having to do a cascading delete.
"Anyone can design a lock that they themselves can't pick"
If you think you have anonymised data sufficiently you may well not have done it sufficiently to prevent others from re-conctructing it: