@shreyan Most of what you are talking about in "ActivityPub" section is only true for Mastodon. #ActivityPub is much more capable.
>identity is extremely tied to your initial server
>Your data is not really portable
ActivityPub specification doesn't say that identity must be tied to a server. Identity can be tied to a secret key or a DID too, and users with different identity types can even communicate with each other.
Specifically, DID-based identity scheme has been proposed in FEP-ef61. Data portability easily follows from it.
So, with ActivityPub one can have everything that ATProto and Nostr offer, and much more.