You are optimizing for a 3 minute fight with no consumables enabled. Mana is going to be your limiting factor. Any gear that gives you more mana is going to be more beneficial to your DPS than +damage, since you’ll get more spell casts. Turn on the consumables you will use - you really should use the consumables that give you mana. Also shorten the fight length unless that is actually how long your fights take, in which case you should do what the optimizer says to get max damage.
It appears you have manually locked in a BiS set instead of following the optimizer’s advice. You would get a lot more damage following the optimizer!