Open source alternatives to classic Nook Simple Touch, such as Onyx?
 

My Nook Simple Touch's battery finally died, after a few years of heavy use. I rooted it, using EBookDroid for reading PDFs and DjVu and the lame built-in B&N reader for EPUBs. Replacement batteries are about as much as buying a used Nook.

Are there any good alternatives to the Nook Simple Touch?

I am looking for one that:

1. has a smaller screen (such as 6")
2. supports an SD card (or at least has 32GB+ of internal storage)
3. is black and white (e-ink technology), perhaps even without a backlight
4. works well with Calibre
5. is open source
6. preferably supports the latest EPUB 3+ standards, and thus can render math with MathML
7. preferably doesn't need rooting
8. possibly supports DjVu out-of-the-box

Onyx readers are nice, such as the Onyx BOOX Classic; however, that one is discontinued. Where could I buy a similar one that meets most of my above requirement in the U.S.?

Your only options I know of are the Onyx or the Boyue (marketed under several names). You cannot but them off the shelf, but I believe you can order them from 3rd party sellers on Amazon.

Thanks to the informative MobileRead wiki, I decided on a Onyx Boox C67ML, purchased with Bitcoin via Purse.io, shipping from Amazon. I hope that was a good choice.

Try installing a deep-sleep battery-saver app on your Nook ST. The battery might be okay. And you should always let it go to sleep by itself (don't press home to make it sleep).