Beautiful !
I'm pretty sure everything you said count. The insulation certainly help. I think my kettle could warm way faster if it was on both doughnut.
I've tried to boil 6gallon with only one 1800w. The sad thing happen that I could only bring it to a simmer after 2hours.
I'm happy with...
Success !
I got myself 2x1800watt (the household limit). Put side by side. I fill my triclad kettle with 9 gallon of room temperature water. After 2 hours I got my rolling boil.
It might be faster if my kettle could fit entirely on the induction douhnut(ring)