I see their repo[0] mentions transitioning to the Pinenote. I'd like to run an ordinary distribution on my Pinenote.

Does anyone know what the mainline support is like nowadays, and whether widely packaged software can make it usable as an ebook reader?

0. https://github.com/Quill-OS/quill

The kernel has mainline support, but it looks a fork is used by most images.

https://git.sr.ht/~hrdl/linux/log/v6.17-rc5_pinenote has many commits.