Actually there are ways to install software on many ereaders. About 10 years ago I used PRS+ on Sony's PRS-500. Nowadays Sony's ereaders are pretty much dead, but there is alternative software for Kindle, Kobo and others. By quick googling I just found:
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.
https://github.com/koreader/koreader
https://github.com/ccoffing/OcherBook
https://github.com/lgeek/okreader
https://fread.ink/
Also:
https://onyxboox.com/ - this is Android so I guess it's bit more hackable
https://the-digital-reader.com/2015/06/19/is-the-booq-cervan...