Tell me if you can steal exploit member base and users emails. That would kill Facebook for good. And you, sorry.
But for posts you need a way to scrape content. Facebook has disabled it quite good, but sure — everything that shows in screens can be copied. That is how AIs and SEO/keyword-companies works.
But there isn’t a legit way for group owners (they don’t owe anything, nor can’t delete a group de facto) to do it.