Is it really self-hosted only?
Yes, by design. A hosted option may come later, but the core repo will always run on your hardware. Your highlights are not training data.
Which devices work?
Anything that runs KOReader: Kobo, Kindle (via firmware), PocketBook, Boox, reMarkable, Android. An iOS reader app is coming.
What about Kindle native?
If you can get the My Clippings.txt off the device, there's a Markdown importer. KOReader on Kindle is the smoother path.
Open source?
MIT licensed. Cloudflare Workers backend, lightweight Hono SSR frontend. Issues and PRs welcome.