You are not excluded from the law as volunteer organization.

As far as deleting old member records and public archives, it depends on what consent members have given before. Membership information, especially receipts for subscription must be kept usually for 10 years for bookkeeping purposes. As for public posts, unless the members withdraw their consent (Excercise Right to be Forgotton) i don't see why. They knew then that the posts are public?

edit: you must formalize this with a privacy policy though, what data you keep, what type of consent (article 6) and for what reason you need it, if you haven't done so yet. Then ask every member for approval of the policy.