Low damage on first time attempted build theory crafting. Help!?

Heya,

I'm having a go at theory crafting a build for the first time but I feel like the damage is lower than I thought it would be. (general idea came from some youtube vids on interesting interactions)

POB: https://pobb.in/B48hDuEhh37I

Idea is low cold and fire res by converting all damage to lightning/chaos, then using Replica nebulis to get big damage multiplier.

Trying to then convert phys to cold, cold to fire. 100% phys to cold from Glacial cascade, and then cold to fire with 50% from cold to fire support and 50% from avatar of fire notable.

I honestly have no idea how to properly use PoB to make a build so I don't really know what I'm looking at, but I feel like the increased damage from nebulis isn't applying? through the conversion? or something?

Open to suggestions of how to tweak the build, new items to try, or just general notes on how to make a build! I'm stabbing in the dark here...

Trying to make this relatively cheap, so items aren't crazy and the char level is only 90.

Any thoughts or recommendations welcome!!