Just a side note: If all you want to do with your ActivityPub request is to be able to followed on Mastodon, you can also do this through some other libraries: GitHub - jehna/mastofeeder: RSS to ActivityPub bridge
But I would suggest to just use RSS. I don’t think it’s a good idea to use Mastodon as an RSS Reader. ActivityPub should not be used as a replacement of RSS. It should be a two-way implementation, like seeing replies as comments under the article or considering activitypub followers as “subscribers” in Ghost, etc. Otherwise I don’t see a value of adding a layer to Ghost a readonly ActivityPub. Instead, I believe pushing bot-like, one-way, non-interactive content to Mastodon and others, kills the potential of it. As I mentioned, RSS already gives that very well.