I recommend going to https://mobileread.com/forums to read about user experiences with Onyx and other makers. They are interesting devices but the support level is not at the same level as a Kobo/Nook/Kindle. I used to have a Boyue Likebook Mimas, which was a great ereader hobbled by a slow processor and itchy software.
https://onyxboox.com/boox_max3
They're 13.3" displays, work with Android and you can use them as an external HDMI display. However, the refresh rate is still poor, not nearly 20-30Hz. The Technology Connections Youtube channel has made two very interesting videos, showing the many caveats of the device (it's an older model, but I'd guess most of it still applies). The main issues are poor software and slow CPU:
https://www.youtube.com/watch?v=7NfX0vlCa4k