E-Ink for O'Reilly Subscription?

Yes. Five years ago, I tried to use an eink tablet with Android to consume the O'Reilly Android app. It tried to animate the page transitions, so it would refresh multiple times for a page flip, making it unusable. It would take most of a second painting one intermediate screen after another. They've since moved away from the page-flip skeuomorphism, but scrolling might still require multiple refreshes. I'm not sure. Using the browser was slightly better, since the browser included a page-down function.

The irony is that the page flip skeuomorphism works for eink because when you flip a page, you expect to go to the origin of the next page once the flip completes. E-readers do this, and it works as long as you don't try to animate it because of how slow eink is. The pgdown/pgup mechanism leaves the last few lines on the screen, which works well when you can animate the scroll action, since your eyes can follow it. But again, eink is too slow for this to work well.

I complained to O'Reilly about this, but they gave me a boilerplate response that they take user feedback seriously. I don't know if it's been fixed.

What I do now is use a Python package that converts O'Reilly learning books to epub. You need a login to use it (of course), and it would certainly be beyond acceptable fair use if you distribute the generated epubs beyond your own devices.

I've been curious for a while about Lenovo's ThinkBook laptops with the e-ink screen (which they've been making for a couple years now). Hoping to find one I could try in the store first...

That would at least get you a laptop-sized screen.

Yes, I use an Onyx Note Air 2 for this purpose. It works nicely, much easier on the eyes than a laptop/Ipad. The Note Air 2 is fairly big - 10.3" - works well for books.