Recently bought the Boox Nova3 and in terms of being hackable it's pretty good. It runs android so you can install anything from the app store. Runs chrome pretty well on the slight ghosting setting. I returned it for the kindle because I couldn't really see my self using it over my phone for browsing and anything I want to read long form I can get there. If you want to go all in on e-ink though I'd recommend it.
It's worth noting that Onyx actively violates the Linux kernel's license by refusing to release the source code. This hampered my hacking efforts as I was trying to build AOSP for the device.
https://www.boox.com/nova3