I own an onyx boox and I can't emphasize enough how much I don't recommend it for anyone. If I had known the company was a serial GPL violator, I would have never purchased it in the first place. My recommendation would be to wait for the refresh of the ReMarkable Tablet, as it runs GNU/Linux and there is a healthy community of people who have made a ton of applications and tools for it [0]. This will likely never occur with any Onyx products and you will be stuck with a device running an old version of Android with SELinux disabled, on a vulnerable kernel, that constantly phones home to Chinese cloud servers.
I explain it all in this comment: https://news.ycombinator.com/item?id=21041543
[0]: https://github.com/reHackable/awesome-reMarkable