I got a Boox Go Color 7 as a less locked in alternative to my Kindle a while back, and overall I've really enjoyed it.
It's apparently rootable, although I haven't done that personally. It's Google Play certified so anything from the Play store works, and side loading Android apps works too. I use it with the open source KOReader app and in tandem with Calibre Web Automated. I did a writeup[0] with some details if you're interested.