I bought myself Onyx Boox Nova Pro, that runs modified version of Android 6. It is fairly expensive, but so far it serves me well and I don't have to think about compatibility/sync much (have Play Books, Amazon Kindle apps installed, and it has decent PDF reader).
But other commenter recently shared fairly in-depth their problems with the device (the build quality can be apparently quite shody): https://news.ycombinator.com/item?id=22614971