Lemmy admin should add option in the account settings for hiding your username from your posts/comments, and hiding your profile from the public, so noone would be able to see all of your posts in your profile page, and even if someone is using a tool for scrapping posts, they wouldn’t be able to link the posts to 1 user, because the username would be greyed out “anonymous” (and would not link to anywhere). Only lemmy staff would know the real username.
ActivityPub makes this impossible. Everything on the fediverse is completely public, including votes, subscriptions and usernames. Even if Lemmy did offer the option, other servers wouldn’t necessarily.
And honestly this is a system that would be mainly used for spam and hate speech anyway. Just make a throwaway like everywhere else.