I briefly tried using ap.ghost.org as my AP target, and couldn’t get past the Rate Limited errors when I tried accessing my Admin settings. My blog did become discoverable on ActivityPub at that point.
I then transitioned to self-hosted AP containers (with the AP webhooks forwarded to them via Cloudflare).
I see that this thread exists and wonder if it’s partially to blame: Question: Can I migrate selfhosted ActivityPub data to managed ap.ghost.org? - #3 by Cathy_Sarisky
(I tried transitioning back to the ap.ghost.org target, but I still get a flood of identity API calls when accessing the Network admin section and end up rate limited)