The optimal build for the great majority of Pistols.

Pistols with high innate Slash can replace PHeat with [Carnis Stinger] for better performance vs high Armor. Performance vs everything else will go down significantly tho.

[Galvanized Shot] is potentially very strong, but is also a highly conditional mod, geared toward high TTK(Time To Kill) enemies.
It requires kills to stack up and status procs to add any extra damage. But the most damning part of how it works is that the bonus damage is only applied to FOLLOWING shots, not the shot that procs the status effect. This generally means that vs enemies that die from less than ~5-10 bullets(depending on the weapon) Galv Shot will perform notably worse, and that's without even taking into account the annoyances of stacking up the buff. Not to mention that Galv Shot is useless vs enemies that are immune to status effects.
So in short: [Galvanized Shot] can be situationally good for high StatusPerSec weapons, but is generally worse than Hornet Strike.

Arcane choice will depend on the weapon. Deadhead for high recoil/precision weapons, Merciless for everything else. (and ofc Dexterity for stat-sticks)

Riven replaces mods with same stats. Toxin, Cold and Multishot being the best candidates.

Good Exilus mods:
PAmmo Mutation - Good for weapons with bad ammo economy, like Kuva Nukor.
Steady Hands - Nice for reducing Recoil on weapons where that is an issue.
Reflex Draw / Eject Magazine - Nice utility.
Lethal Momentum - Good for weapons with damage-falloff, but useless on all others.
Ruinous Extension - Only works on Beam weapons, but is generally the obvious choice for these weapons.