Fire/Phys/Trauma Commando Build Advice

So as mentioned in another thread I want to start understanding and theorycrafting my own builds, rather than just blindly following somebody else’s.

I figured I’d go with commando for my first attempt as demo and soldier do have some crossover with physical and internal trauma. My aim is to have fire as my main damage type, and physical/internal trauma as my secondary/tertiary damage types.

Please don’t just link be somebody else’s build or just say “change this this and this”. Please explain to me WHY something should be changed. I know there’s probably a lot wrong with it as I was mostly taking guesses (especially with the gear as I don’t have many legendaries at all to have actually played with).

Grimtools link:
http://www.grimtools.com/calc/62aDWxN4