I had the same issue where every couple of days, the Fire HD 8’s brightness would dim to 5%. I changed two settings and am not sure which one was the fix… 1) Set the Fully Kiosk Screen Brightness under Device Management to 190ish. 2) Enable Developers Options on the Fire HD 8 and enable the “Stay Awake” setting. Fully Kiosk will still “sleep” according to your Screen Saver settings.
To enable developers mode on the tablet… Settings>Device Options>About Fire Tablet>Tap the SN 7 times.