On one hand, there's folks using the vendor's OS, which looks like an abandoned custom-made Linux distro, where compiling anything is a nightmare. It's not really made to be hackable, but just to read books purchased via their partners.
OTOH, there's a near-mainline kernel, which works nicely, and hacking on the device becomes much nicer, but e-reader-oriented software seems to commonly rely on kernel APIs only present in the vendor's abandoned fork, and won't work on a more mainline Linux. But this is a great target for actually developing stuff.
I'm tempted to write my own minimal e-reader-focused DE, but honestly, I don't want another project on my hands and would like to use something that's already there.
Like, in my mind is about as dumb as an electronic device gets. Load epubs, a few options for font size and margins, you're done forever.
I don't intend to use the e-reader to displace paper books, I have no quarrel with those. I mostly want to displace "sitting on my desk, reading a multi-page article on a computer monitor".
It's custom software but still pretty dumb, doing most of its work in the background.
On the plus side, with your setup, you can have the lowest friction ebook experience possible on planet Earth by installing koreader, and then the z-library plugin.
I sold my color eInk device after trying it for two days, and went back to B&W.