I think what Ghost (as you mention) is trying to do here with v 3.0 is interesting - seems to have most or all of the features of Substack - easy to set up on their hosted option plus full ownership of the email list and open source / potentially self hosted.
Interesting that "The Browser" is on Substack (one of the top 5 newsletters) but the affiliated and more recent "The Viewer" is on Ghost.
Of course Ghost still has a relatively small ecosystem compared to Wordpress which I think is why Ben Thompson still expressed a preference for a Wordpress based setup.