(update: this was a terrible idea. Please refer to latest solution in this article: How to write articles in Markdown for Substack and Medium using GitHub Page’s Atom/RSS feed)
Substack’s editor supports a good set of HTML.
The plan is to automate a headless browser with a Python script and just type character-by-character using the plain-text markdown.md as source.
Images and all seem to be possible since it is possible to paste rendered HTML into - at least - Substack’s editor.
Article’s change history would be available on GitHub, within the repository; in Substack, I might keep just the latest updated version (latest version from GitHub).
Updating other channels: Dev.to seems to update from a Substack feed automatically; Medium refuses to import from Substack, so I might employ the same character-by-character technique.
GitHub pages seems to be an amazing place to have all pre-prints available online, in a public repository.
Collaborators or ideas welcome!
PD This was written directly on Substack for now.