Ad-free is not necessary for running SharpTools dashboards on those Fire tablets. You can use Fully Kiosk Browser (download the APK specifically for Fire OS) app, which is a very popular app to run dashboard on Fire tablets, and many users have configure it to dim the screen in screensaver, and exit screensaver when a motion is detected by its camera. So it simply doesn’t go to the lock screen.
Here are some basic Fully Kiosk Browser settings that you might be interested.