One thing I forgot to add regarding Infernal Brimstone;
It’s not a whole RATA item, it’s not a caster item, and it’s not a proper AA item. Since first two has more proper options; I think it should have phys>fire conversion.
You can’t get that conversion from any sort of gear or sets so it should be the weapon. For FS it’s not needed much but it’s a 2H Mace that focus on Burn damage so clearly RF line is more suitable for that matter and because of no conversion(As you can see above with my build) you end up with stupid amount of unconverted physical and trauma damage.
Build already not that great, especially when it comes to hitting with your big 2h fiery mace. Having some %40-60 phys>fire conversion will be a good buff and won’t make the build owerpowered. Or simply you can add a Righteous Fervor modifier to it that converts physical into fire.