Long shot is only good when used from long range or when used to detonate. If you have any stamina problems turn it off, or jump to control him and proc combos manually. Or I think if you set him to follow your mage or shield fighter he should be more likely to shatter or rupture.
His cooldown reduction passive is very strong, but only if you stack critical chance on your other party members. This means he goes really well with a 2-handed battlemaster or a reaver (because their passives give you another good reason to stack critical), a storm mage (because energy barrage and lightning cage both generate lots of crits), with an artificer inquisitor (who buffs crit chance for everyone), or with other rogues using crit passives.
He has a very weird interaction with fire mages and necromancers. Damage over time damage spells don't crit, so these mages usually will not want to stack critical chance. However, both trees have panick skills. There is a rogue passive that allows auto-crits against panicked enemies, so Varric can go well with Dorian and a stealth inquisitor or with Cole and a necrofire inquisitor. The fire tree also has the Flashpoint passive, which rewards building critical chance. However, it's probably better to just take that passive on a more generalist mage build rather than specialize in fire.
Because of his cooldown reduction, he can spam his skills until he runs out of stamina. Cooldown reduction works best with skills that have short animations, because otherwise you might not have enough time to use all your skills. With the passive that returns stamina for each crit, he can spam multi-hit skills basically without ever running out. This means that throwing blades and leaping shot are his best damage.
The best bianca arm schematic before Skyhold are in a cave in the southeast Hinterlands. The absolute best arm schematic is in a dwarven tomb in the Hissing Wastes. It's not too far from the starting point and you can sneak past most enemies if you're low level. The next best schematic can be bought from Bonny Sims for a reasonable price. The best grip is in the elven ruins in the exalted plains, but Bonny Sims sells one that's almost as good. The best aiming module is in another Hissing Wastes tomb, but against Bonny Sims sells one that's almost as good.
If you're willing to control Varric directly, you can ditch Bianca and equip daggers. Stealth into melee, use the spike trap without breaking stealth, and then spam spinning blades & elemental mines until everything is dead. You can also play Bianca melee with stealth, hook and tackle, your traps, and leap shot, and maybe also sleep powder/mercy kill.