I have been using the tool, which I called Artemis, for several months. Every morning, I looked forward to my "morning paper" of blogs I love reading.
There are no notifications, read vs. unread states, counts of posts, etc. Only the last seven days of posts are available. The colour scheme is changeable. Dark mode is supported. All popular feed formats are supported.
There is no reading interface to read blog posts; rather, the links take you to the authors' websites. Many of my favourite bloggers put a lot of effort into the design of their blogs and like to change things up; I wanted an experience that embraced that.
The reader is now available for anyone to use (with invite code "hn").
I believe you can RSS most of them. certainly all the substacks.
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 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.
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...