I haven't open sourced the Backend yet, but the reader frontend repo is fully working without the Backend. The Backend is used for emailing yourself and doing file conversion on the Backend instead of the Kindle to save battery.

So without the Backend you're only missing the email feature. All you need is a proxy to bypass CORS.

To be honest I was thinking of keeping the Backend closed source to add subscriptions ($1 per month) to cover the hosting (4$), which means I only need 4 users to break even haha.

Will think about open sourcing the Backend and get back to you.