Well considering your setup I will offer my advice cause I run the same thing:

Use Hex as often as you can (Even more then one target when you can).

Use your DK Bone Shield Proactive before large pulls and it also helps if you know what the mobs actually do (comes with time of running it so often). I know with my DK there are times I use Ice Bound Fort (Glyphed) when pulling 4+ trash mobs. Most of the time I burn one of them down the moment they reach my DK so its usually not an issue but never the less. Also don't be afraid to use TS on Round Robin. If you drop DnD and all the trash is inside of it, they will stay aggro on your tank for sure. Use TS to get some breathing room and get some extra heals on tank/dps whoever needs it.