You need lots of DoTs and a good rotation, including buffs from group members.

Dropped 700 SP to switch to Twice-born and can now do an extra 5-8k dps with a good group and all the available magicka NB DoTs.

If you're not already aware, Tamrielfoundry is handy for good builds including suggested rotations, and @Alcast has guides on animation cancelling, rotations and their correct order I believe.

I'm guessing rotation, though 5k buffed is a bit low - should be able to get around 7k without too much work. Stamina is pretty good though, so what you probably need more is damage effects - armor penetration / ignore armor, damage buffs, along with crit (you don't mention your crit stats - very important in PVE, not so much PVP). Light / Heavy weaving, where you alternate melee strikes with casts, jammed as tight together as you can.

At the top end, Everything Matters. How hard you hit, what effects you have on the mob, your max stamina (OR magicka - going both ways will only make you hit for less) and therefore food, champion points to round out the stuff you don't have in gear. Spell rotation to take advantage of those 3-5 second debuffs and hit them with the big guns. Ultimate - using the right one? Generating it fast enough to use it often?

I hesitate to mention this, but... Deltia has some of the... most detailed.. build videos for just about every class. He knows stuff, no doubt about it. I only hesitate because he tends to like to do things the hard way on occasion. He's very detailed, and his style may not be yours, but what his videos do get right is showing you what to look for when choosing abilities and ability rotation. Major Brutality, Empowerment, Bleed... when to take advantage and what abilities give you the most bang for your buck. What's glowing where so you don't have to watch for an icon to pop up when something procs. Even things like holding your melee button as you crit rush in, so you can release it in a heavy attack right as you get to the mob. (Note: He has a LOT of videos dating back to the beginning of the game. Even the older ones have some good material, but you'll want the most recent for your class to take advantage of all the changes over time.)

Maybe you know most of this stuff, but I learn something new just about every time he puts out another video. If nothing else, its fun to try new things and maybe find something you can adapt to the way you play.

Hope this helps!

You need a decent rotation for great dps. Gear matters alot, but you can easily get 30k dps with just crafted/bought gear.

You can try a similar rotation to this one, just replace the sorcerer skills with your dk dots