Regarding Onyx: I don't own one and have not systematically researched them. But, I've seen posts here and there mentioning that they "churn" their hardware (i.e., release e-readers constantly but drop support for the older ones just as constantly). Similar posts mention they don't update their Android version and when they churn from one generation to another of a device, they still don't necessarily update the Android version:
https://blog.the-ebook-reader.com/20...eady-hates-it/
Again, I haven't systematically researched them.