To date, I’ve only used the Kindle notes site (https://read.amazon.com/notebook), where I copy the plain text notes out and then paste them into Notion or whatever other writing app I’m using at the time. Though that’s with books; with web articles, I tend to read in Instapaper. Do web article highlights show up in the Kindle site as well?

A less manual option is Readwise, which syncs Kindle and Instapaper highlights and can directly sync them with Notion (or Evernote and Roam Research). Seems like a popular tool, though in a recent Capiche discussion about Readwise everyone seemed undecided if it was worth its price. If your core use-case is wanting your highlights in Notion, that one feature may be worth it.

Another option I tried in beta but haven’t tried recently is Klib, a Mac app that syncs Kindle highlights and turns them into Markdown text, which you could them copy or import into Notion.

Readwise just launched a beta of a Notion integration!

Readwise offers a Notion integration. It looks astonishing! Very beautiful and very useful. It’s expensive, though.

I've struggled with this as most of the options are mainly for highlights made on books bought through Kindle, whereas most of my Kindle reading is of epubs/pdfs downloaded elsewhere.

Latest option I've found is this, which I'm about to try but looks good: https://github.com/paperboi/kindle2notion