I've had my GTP 8.4 since launch. It has always had poor battery life when running stock roms. Sometimes it does better sometimes worse. Even without apps installed there is no difference. Android system is almost always listed as consuming the most battery. That is the biggest draw back on all reviews you read on this tablet. Like you have done, I have installed numerous battery saver and wake lock detecting apps but to date have not figured out a way to better the battery life at all. I have done factory resets, rooted, gone back to stock, it's always the same with stock roms. I did, however, found that when running Cyanogenmod the battery did last a lilttle longer.
Bottom line is, the battery installed is probably not big or good enough to handle the display plus everything Touch Wiz needs. I got used to expecting to having to charge daily with moderate use. I can maybe go 3 days with light use (about an hour screen time).
I have the original Ipad Mini as well, it has issues sometimes especially whenever IOS is updated. But then settles down and keeps running almost as good as it did when new. I'm not saying one is better than the other, I love Android (not my GTP) because of everything you can do with it...maybe there is different hardware out there that would meet the battery life you need.