First of all, welcome to the forum 
If you have BiS armor pieces, by all means use them (unless for some reason your resistances or DA suffers too much due to it). Even if you do not have the full sets, they tend to be quite powerful in their own right.
As for farming SR, if your char has reached level 94 (requirement to wear the endgame items and sets), head towards SR. You can finish the rest of the Campaign later. Exception is if you need to farm faction reputation to get access to faction items and augments.
As for DW Pistol build, I am primarily a Pet player/builder. So I do not really keep myself updated on what the best non pet builds are in the current patch. So I fear I won’t be much help there.
You can however, start a new thread and ask for suggestions and someone should be able to help. Alternatively take a look in here and see if anything strikes your fancy: ⚔ Nery's beginner build collection