Update: This WA should no longer be needed because the recent versions of DBM should have the auto-marking working properly (consistently used raid markers for each wave of adds).
Only one raid assist should run this WA.
Must be in range to see mob name plates for auto marking to work.
Applies marks in the same order that DBM does.
Load condition prevents this from loading unless you are the Main Assist, adjust as needed.
I didn't want this thing to go active by default for multiple people in the raid running it with assist, because it will conflict with itself, just like it will conflict with DBM if any other raid assists still have the auto-marking enabled. I hope that DBM fixes their Shadow Beacon auto-marking soon.
This Plater script hides the enemy name plates of the adds without Shadow Beacon:
Go into /plater, Scripting, hit Import Script icon next to question mark icon near the top-left, paste and import.
Modified version of WA from ThatVay: