Help perfecting my Chieftain 100% fire conversion Voidforge build

Hey all,

I started this league with a few friends as an in-betweener between casual and hardcore. I had a level 91 deadeye (Doomfletch) within ~1 week and I got that to killing shaper gaurdians with Minor ish issues (depending on the map mods). I didn't particular enjoy it however as it just felt like a build I'd copied. So, with the currency I'd accumulated alongside liquidating all the wealth in the gear that character had, I decided to make my 'own' build.

So, preamble aside, here's the build low down:

I'm playing a chieftain to capitalise on the fire damage buffs and most importantly: https://pathofexile.gamepedia.com/Ngamahu,_Flame%27s_Advance (50% of physical converted to fire).

Alongside this, I'm using a Xoph's blood, which gives me the avatar of fire keystone (https://pathofexile.gamepedia.com/Avatar_of_Fire)

50% of Physical, Cold and Lightning Damage Converted to Fire Damage
Deal no Non-Fire Damage


Finally, I'm using Voidforge. (https://pathofexile.gamepedia.com/Voidforge)

Voidforge has

"Your Elemental Damage can Shock
Gain 300% of Weapon Physical Damage as Extra Damage of an Element
Deal No Non-Elemental Damage"

As we have 100% physical to fire, we don't "lose" any physical damage from the physical portion of voidforge. However, we obviously lose 50% of cold and lightning if the random element is either of those (only 50%, as the Avatar of Fire keystone converts 50% of cold to fire and 50% of lightning to fire). We can achieve 100% cold to fire with the cold to fire support gem, and we can get 40% lightning to cold (which results through transitive conversions to 90% lightning to fire) with the following ring: https://pathofexile.gamepedia.com/Call_of_the_Brotherhood

All in all, we can efectively take the average physical damage of Voidforge and multiply by 3.97 (the .97 because 1 in 3 hits will be lightning where we only get 370% damage instead of 400% damage because of the 90% lightning to fire caveat).

So, we have a 397% multipler on the voidforge average hit DPS, with 66% fire penetration from Xoph's (10%), Ascendancy (10%), Talents (10%) and fire penetraion support gem (36%). We also have "your elemental damage can shock" and I've got ~25% crit from the tree, so I should have constant uptime on a (presumably) heavy shock. There's also a nice synergy with Kaom's heart in this build, with the 40% fire damage being a little icing on the cake.

Despite all this, the build feels underwhelming. I'm currently level 84, and whilst I can do tier 13 red maps fairly comfortably, it seems my damage output isn't as high as it should be given the gear that I have (Kaom's, Xoph's, Voidforge and some nice rares) and the investment I've put in. There are videos of people doing these bosses in like 1 hit, and it takes me a good 5-30 seconds depending on the modifiers.

On the upside, my defense is strong: 6.8k hp, 4 endurance charges (will be 6) and ~1.1k regen/second with ~3% attack damage leech at level 84.

I started leveling this build using an Oni-garoshi, and I'm worried the oni will simply be a better choice.

My character can be found here: https://www.pathofexile.com/account/view-profile/SwagOverloadYolo/characters (CastOnStupidBeast)

A (crude) pastebin for POB is available: https://pastebin.com/7WHfr0aA

Looking forward to hearing your constructive criticism (or even whether my idea will fail due to x/y/z reasons!).

Thanks!