It seems to lean more into transactional email than being a 1:1 Substack clone, but I have heard good things about Dittofeed [1]. It's pretty self-hosting friendly, all things considered. [2]

We went with Substack basically because it's really easy to newsletters live on as webpages, but if you're going through the trouble to self host and build a website anyway, Dittofeed might be a good fit.

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[1] https://www.dittofeed.com/

[2] https://docs.dittofeed.com/deployment/self-hosted

https://ghost.org/