Below is a screenshot of my UI, hopefully it will help give you an idea or inspiration for how to layout your own UI to be effective for raiding.
Click the image or visit the Full Image Link to see the Full Resolution UI Screenshot.
Dispersion Cancel aura
I don't have my cancel aura on my dispersion keybind as its possible to accidentally double click the ability and cancel when you don't want to. Instead, I use it on my mind blast / Shadow word: Void.
#showtooltip Mind Blast
/use Mind Blast
Power word: Shield mouseover Macro
#showtooltip Power word: shield
/use [@mouseover,help,nodead][help,nodead][@player] Power word: shield
Shadow Mend mouseover Macro
#showtooltip Shadow Mend
/use [@mouseover,help,nodead][help,nodead][@player] Shadow Mend
You can find a wide variety of useful weakauras for Shadow Priest at https://wago.io/weakauras/classes/priest/shadow