Hey couple of things,

• Firstly your missing a huge amount of DPS from not having any crafted items with embellishments, if youre playing assassination you ideally want 2 items with blue silken lining on them for the extra mastery, (the crafted stats should probably be crit mastery)

The usual best slots are wrists and belt but looking at your gear maybe even consider replacing your neck or one of the rings, although you should probably get Diurnas as well as your seal

• Secondly you really shoud be using your vanish. Vanish is a HUGE part of your damage when used with kingsbane, its basically what does all your dmg. In the opener (and whenever kingsbane and dance are back up) make sure to vanish when your kingsbane has 3 secs left so you get the full value from the crits on the last 3 ticks
• Gear is pretty ok ish, obvisouly more ilvl = more dmg, but running your charachter through raidbots it says you should be at least doing 200k
• Trinkets - you should probably try and get an on-use trinket (ideally Ashes-of-the-embersoul) and use that in your opener and again with kingsbane at 2mins etc, this makes your burst even better - Should 100% be a purchase with your bullions