“The content” on all Lemmy instances is the same. There is no account migration, but you can just sign up on lemmy.ml. If you already had an account there and you want it back… I don’t know if it’s possible for an admin there to restore it, you might have to get in touch with them.
My (somewhat) hot take is that large migrating subreddits should probably host their own communities, which is what we did when we told people on r/PrivacyGuides to move to Lemmy. Or at the very least, actually coordinate with instance admins beforehand about all of this, clearly lemmy.ml isn’t the ideal choice for this situation.