May be your battery is OLD? Try changing it. Remove your microSD card. Try a few things. or you can actually install a clean non-amazon spy-free open source software by complete unlocking. Then you have only a minimal apps - no crapware. See below:
* need to install ADB drivers
* Boot into recovery: Hold Volume Down button & Power button at the same time
* …
'''Carefully read this thread/post:'''
https://forum.xda-developers.com/amazon-fire/general/unbrick-amazon-fire-7in-2015-5th-gen-t3285294
Post here if you are successful.