There are multiple custom roms for the Mini M8S II - both Marshmallow and Nougat, including AndroidTV builds.There are at least 4 custom roms on FreakTab. They are much better than stock. You can find the firmwares over on Freaktab here:
In addition, if you check on 4pda under the Mini M8s ll section for additional firmware versions. (use chrome/chromium for the auto page translation - works great!)
Only other thing to consider is - how long ago did you buy the box? With the regular s905 box, there were several revisions of the CPU. The early revisions had hardware bugs in them, but this was eventually corrected. Not sure about s905x.