I actually question whether GDPR is up for the task of distributed systems like this.
Like, if you put in a right to be forgotten request to your host server, it's not at all clear that they're responsible for the copies of your content that are being hosted elsewhere, any more than asking a news website to remove your personal information from an article requires them to also hunt down anyone else who has copied and spread the story to remove it, too.
Different Lemmy websites are independently owned and operated, and your local admin holds no authority over other admins. They can request deletion on your behalf, if that's a legal requirement, but they cannot compel action. I'm not even sure they can act as your proxy, given that there's no formal relationship between admins.