Suggestions on soldering through hole large planes
So I decided to go with what @WhatRoughBeast suggested. I purchased a Hakko FX-801, with a few of their large heat capacity tips for around $1100 total. It has a power output of 300 watts.

I have assembled 5 boards with it, and it is truly incredible. It can heat up any component including my large heat sinks and buss-bar to soldering temperature within 5 seconds.

Highly recommend this iron to anyone in a similar situation