It's a shame there's no good hardware ereader to use it with. Every e-ink device is closed hardware, runs only closed software, and is trying to lock you into some DRM-rich ecosystem.
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.
The bq cervantes[1] is completely open source. It runs an old (old) Debian Linux, but it's OSS top to bottom, they tell you how to get into it and run your own firmware, their e-reader app is open source, and you're free to hack it apart as you see fit (koreader has a target for it too).
FWIW, I've never had a problem with my kindle, which has never been connected to the internet.
Even the 'special offers' dissipate after being in airplane mode for a certain length of time. I see no value in using any online feature—I've never taken my kindle off of airplane mode, and use Calibre to load all content.
I wouldn't have paid for the ad-free version of the kindle had I known it was just a lock-screen promo for a book. I'm exposed to so few new books in my life that I kinda like the feature. Then again, I also wouldn't have bought the ad-free version had I known I was going to keep it in airplane-mode since the day I bought it.
Back when I first bought my kindle and made this decision, I was thinking the ads would be something like a "checkout these deals on Bose speakers!" interstitials every three page turns.
It's a shame there's no good hardware ereader to use it with. Every e-ink device is closed hardware, runs only closed software, and is trying to lock you into some DRM-rich ecosystem.