So the first thing to think about when it comes to simming is: That’s the best possible amount of damage you could do in a vacuum, with everything lining up perfectly. Sims take in account every buff available to you and assumes they’re up 100% of the time, and assume you crit for every single hit, and then calculate your damage. But at the same time, you shouldn’t be doing half of what the sim says you should be doing. I think the issue here is the type of sim you’re running. When you select fight types, remember that there’s only 1-2 patchwork style fights in Nyalotha. So selecting that as your sim option isn’t going to give you accurate results. Most of Nyalotha’s fights are either Light Movement or Hectic Add Cleave.
So baseline you want 3x Expedient IIIs, and 3x Severe IIIs. What you add from there depends on a couple of factors. Do you have more than 40% crit without the severes and you’re now sitting at 50%+ crit? If so, you need to swap out your azerite pieces so you have 3x Spiteful Apparitions, 2x Chorus of Insanity, and 1x Whisper of the damned. Secondly, dps proc corruptions such as Gushing Wound and Infinite stars will give you varrying results depending on the fight. Solo target fights like Inquisitor or Shad’har will work extremely well with infinite stars, but on fights like Carapace or Hive Mind, it’s going to be much less useful. Corruptions such as Severe and Expedient are good for every single fight, so when in doubt, add more severes.
Again, this depends on the fight you’re on. But a safe bet all-rounder build is Memory of Lucid Dreams/Condensed Life-Force (sim yourself to see which is better) for your main, then breath of the dying, blood of the enemy, and essence of the focusing iris for your minors. Keep in mind that each fight is unique and if you want to min max you need to change them on a fight-fight basis. Otherwise, just use the general build I just listed.
Looking at your gear, you’re doing pretty well. You need to swap your humming trinket with the vita, that should be your #1 priority at the moment. You need a little more haste, but that’s just me eyeballing it. Run your stat weights sim to see which benefits you the most. Most Shadows are running at least about 32% haste. Additionally, swap your corruptions over to the list that I mentioned above and you should see a clear difference in your damage.
Tips to get better: I’m by no means a top tier Shadow, but the thing I find I have issues with is planning. Shadow is a very quick, punishing class, and if you don’t plan ahead for later in the fight, then you’ll find yourself with your pants around your ankles. For example, in N’zoth the corruptor, when you’re being pulled to the front or back and you get stunned, this is an excellent spot to use Dispersion in order to keep your void form. This takes precise timing, and you need to plan for it. Shadow requires a lot of creativity when it comes to how we fight in raids.
The other big thing being is you always need to be doing something. Even if you’re not in Void Form and you’re having to run, spam Shadow Word: Pain. This makes it so you’re still doing some form or damage, and you’re building up insanity so you can get back into void form and have Void Bolt again.
Keep at it and don’t be too hard on yourself. Shadow’s one of the harder dps in the game.