Check other builds made by me for the full loadout

In-depth Void Cascade Lvlcap guide included under the Ash Prime Build

You do NOT need a riven. Riven for any of the weapons in this loadout is an overkill.

Replace riven with Galvanized Savvy

Best riven stats: +cold +tox +ms -cc OR +cold/tox +sc +ms -cc
Positive stats to consider: sc, sd, recoil, fr (personally wouldn't care about sd, recoil and fr)
Status Chance allows for more viral procs; Status Duration allows for longer lasting viral procs (operator amp damage is increased by viral procs); Recoil/Fire Rate are self explanatory.

You don't need to use the incarnon form, the base form should be oneshotting thraxes at levelcap if aiming for headshots and 3-4shotting with bodyshots assuming your Xata's Whisper is at 50+% and they are armor stripped.

Evolutions:
+50% Projectile Speed
+50% Magazine Reloaded/s when Holstered
+20% SC -10% CC
50% chance to deal 2000% Damage on non-critical hits

Don't reload with this weapon, throw your Glaive Prime/Cerata when you need to reload and your evolution and synth mods should restore all of your ammo.

Throw your Glaive Prime/Cerata to keep Primary Dexterity at max stacks.

For long range kills make sure your Laetum stays stacked in its incarnon form.

You might struggle with ammo for this weapon if your squad isn't killing enough mobs, you can either swap between [Laetum] and [Felarx] for thraxes or bring [Shotgun Scavenger] as an aura.