nucleative put 2 different ideas in their comment actually.

The first idea was moveable accounts, just transferring like Mastodon does, which still gives the instance admin a chance to decline or ban you.

The 2nd idea was the account working on all instances at any time by using public/private key encryption, this could cause issues with moderation.

I guess you could combine the ideas to require an explicit transfer, while still using the encryption keys to maintain ownership of old content in some way, maybe.

Lemmy's export/import settings isn't really that different from what Mastodon does, since you normally can't follow Lemmy users anyways, so there are no followers (or very few) to transfer.