Kindle-only right now as it's the biggest piece of the pie and what I use personally, but let me look into the KOreader highlights storage format and see if I can hook you and the other KOreader users with something.

Can you summarize how it works? You highlight something on kindle and it gets sent to your online db for storage?

You make your highlights on a physical Kindle or Kindle app as usual, and then go to https://read.amazon.com, select the book that you want to import the highlights for and right click to import them.

There are other services available that will take your oauth login details for your Amazon account and spin up something like Selenium on their end to scrape all of https://read.amazon.com after logging in on your behalf, which is a bit more like what you're describing (it's not instant, but scheduled), but in my experience this sacrifices a lot of flexibility (deciding which books you want to import highlights for) and reduces the quality of the content in your data store over time. The other downside of this approach is that you need to reauthorize the service every couple of weeks or so.

Ah I see, initially I thought it was something like a plug-in for kindle and it just does this automatically but I guess they don't allow such things.

Thank you, I am currently struggling to navigate in the world of RSS feeds, bookmarks, notes, reminders and everything else. I will check your app out if you don't mind.