My Onyx devices with Android 6 are working quite fine for my limited purposes. I have also been able to install and use most of the apps which I need such as Moon+ Reader Pro. I've even been able to install a good number of apps which I don't need, but which are nice to have.
If you are experiencing difficulty installing and using certain apps, you may wish to look on sites such as F-Droid for earlier versions of those apps which are compatible with earlier versions of Android:
https://f-droid.org/
I have done that with a couple of old tablets which are running Android 4 and with my Hisense A5 phone which doesn't have access to Google's Play Store. Very easy to sideload the apps.