FBReader is quite nice for reading offline ebooks on Android.
I realize that you are griping about dedicated e-readers, but I want to make clear that its not strictly an Android problem. The more open Android devices work just fine as e-readers.
My flow is basically Calibre -> managed library -> android phone -> offline reading anywhere.
I have 2 apps that I use on my android based eink reader. I mostly read fiction so those 2 our enough for me. Turn on wifi every 2-3 weeks to transfer the books from calibre to calibre companion.
None of this is a replacement to a properly open ebook reading device which does not exist.
On the software side, openinkpot existed at some point, but the project died.
I'd love a properly made piece of very low power open hardware running e.g. nuttx rather than boring Linux/Android.