One nitpick about ActivityPub actor identity — the username doesn't have to be part of your ID (the URL that points to the JSON object representing your actor). It is in Mastodon, but some other software (Smithereen that I work on, and also Misskey) uses opaque identifiers derived from database row IDs. This allows for cleanly changeable usernames since you can just update your `preferredUsername` and `url` fields.
Your instance is in your username. The full username is "@grishka@mastodon.social". You use WebFinger (https://mastodon.social/.well-known/webfinger?resource=acct:...) to convert that to the ActivityPub actor ID, which in my case would be "https://mastodon.social/users/grishka".
And yes, that's one weakness of this system — there is no "real" account migration. Most you can do is set up two-way references between your old and new accounts and ask your followers to unfollow the old one and follow the new one. But your past content doesn't carry over.
But then idk, Bluesky's identity service is completely centralized, so the fediverse is better with regard to independence?
It still doesn't solve the account migration issue though right? If you move to a different instance your old instance still has to redirect.
How do people find you on mastodon if your instance isn't in your username anyway?