You’re posting on public forum. If you don’t want Facebook to see it then stop posting. Defederating isn’t going to change that. It just stops the content flow from their instance into yours. Not the other way around. For them to get data other than your content and upvotes you need to install Threads app because that’s only available to the admins of your instance and Facebook can’t get it wether you federate with them or not.
People are demanding for defederation but almost everyone is confused about what it does.