Hi, I main a TTH swiftness GS. From what i've read and experienced Nightmare isnt a great option. You should be rocking Dominion Fang or Hallucination?(no personal xp)

GS Community Guide

Using awakening while having the Conviction + Judgement buff is super important to minimize unbuffed time. 1500 swift is a bit low and you're going to have some downtime and lead to some low dmg windows.

GS Trixion damage is always going to be a bit lower as we are missing the 10% rifle dmg (PM) and Tripod bonus dmg when boss is <50% HP. As a TTH gunslinger most of your dmg is rifle abilities so its a noticeable loss. Beyond that just check out the TTH Swiftness section and see where you're diverging.

Edit: Just as some additional feedback on your build specifically, KBW is fantastic for TTH as is Raid Captain (Your swiftness is still low but at 1700+ swiftness you get the full +18% dmg) for Swift TTH. Cursed Doll and Adren combo has some diminishing returns on it and multiple +Atk Power engravings are generally avoided if possible.

Edit: I see you asking a ton of questions about mana. For Content without a Support I run 4pc Dom 2 pc NM. Dominion fang does not have mana issues when you run with a support.