wow. thanks for all of this - i'll be noodling about it. the model you've described is interesting & it's clear you've put a lot of thought into it.
All of the "recent highlights" feeds on my website are built around this idea (I save interesting "content" [with the URL/book/comment permalink being treated as metadata instead of a primary identifying key], categorize it, and based on the categorization, the content is automatically published to topic-specific RSS feeds which can be consumed by individuals with RSS readers, or, in this case, my website!)
There is a 30 day trial and it's pretty cheap after that ($1.99/month), but if you want to try it for longer than 30 days, send me a DM on Mastodon and I can extend your trial for longer as well.