But I could be wrong
Substack is also a bit of a pain to integrate with because they have zero useful contact information and direct all inquiries to a chatbot that is beyond useless, makes it so you have to guess how they want you to interact with their servers since there is nobody to answer questions.
[1] Preview of my take of the idea: https://mastodon.social/@marginalia/113670235590972416
I built a site around programmatic blogrolls (sites that publish an OPML file of their recommendations using a specific format). I collected a bunch of initial subscription lists[1], which are curated lists, "planets", and web rings. The I spider out from there. The aggregate list is too diverse, and definitely has some low quality content, but connections between blogs are useful for discovery [2].
[1] https://github.com/robalexdev/rss-blogroll-network/blob/main...
[2] https://alexsci.com/rss-blogroll-network/discover/feed-ff6cc...
I believe you can RSS most of them. certainly all the substacks.
I think OP’s project is a nice place to potentially have some default feeds. Both for purposes but also because it’s nice to see some interesting content once you sign up. Maybe even just major news items.