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").
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