As an admin and moderator in one forum for 25 years and a visitor and active member in around 200 other forums over the years, I can say that other forums never delete users' knowledge or specialist posts, as this is precisely what makes forums attractive.
When other forums want to clean up, they try to get rid of inactive members, for example. They are notified and told that if they do not log in within a specified period, their account and posts will be deleted.
This alone would remove a lot of ballast from Zooville. Banned or otherwise blocked users and their posts could be deleted without further ado. A similar procedure could be followed for personal ad and meetup threads and posts, but with a time limit. Thread creators are notified when their thread is one year old that it will be deleted and that they should create a new one if necessary.