Ian, I am really intrigues about the rise of Substrack. I liked Sean Monahan concern about the magic of micropayments:

A new micropayments platform for newsletters won’t magically liberate public intellectuals from commercial pressures; it won’t solve the tensions between free speech and safety; and I highly doubt it will make having a career as a writer any easier. But it will create space for writing not tailored to the trending on Twitter section, encourage writers to develop a deeper relationship with their audience, and promote the sort of writing (both longform and short) that doesn’t fit neatly into the categories of legacy media.
Sean Monahan https://www.theguardian.com/commentisfree/2020/nov/17/substack-media-platform-publishing

I really liked your association between Substack and Medium. What Monahan labels a ‘social media interregnum’. I really liked Chris Aldrich’s point about ‘yet-another-platform’.
I have been using Buttondown, but have reservations and am considering moving to Mailpoet, especially as all my posts are already on my site.