If you are looking for a simple, stable ROM for your Samsung Galaxy Tab Pro 8.4, you should check out the Vanilla Unicorn android 6 Marshmallow ROM. Since this is vanilla, there is no Samsung TouchWiz user interface onboard that will slow down your android tablet.
To flash this Samsung Tab Pro Marshmallow Firmware, download the ROM from here and copy it to your tablet’s microSD card. Also copy the gapps since this does not come with any. Then, go into TWRP Tab Pro 8.4 custom recovery and do a full wipe. Then, flash the ROM followed by gapps. Now reboot and enjoy the Android 6 Galaxy Tab Pro 8.4 ROM.
Here is the feature list of Vanilla Tab Pro 8.4 ROM:
Status Bar (battery, clock, date, etc)
Hide superuser icon
Disable heads up
RAM usage bar
Lock screen tweaks
Nav bar customization (dimensions, layout, etc)
Hardware keys actions
Ambient display and wake gestures
Since the ROM comes pre-rooted, there is no need to root it again.