One thing I keep coming back to when comparing #ActivityPub and AT Protocol: ActivityPub is basically a convention on top of the #web stuff we already have. An actor is a JSON-LD document you GET. An inbox is an endpoint you POST to. If you already run a website, you can bolt this on without changing the site's basic shape. Ghost didn't set out to join the #fediverse, and then years later it could, just by adding an endpoint.
AT Protocol feels less like that to me. Running a PDS means a signed repo, a Merkle search tree, a firehose, a DID. It's not a layer you add to an existing site. It's closer to standing up another backend beside it.
I think that's why ActivityPub keeps making sense to me. You can join later. You don't have to have built the whole thing with federation in mind from day one.