Question about Mods and Damage

So my question is related to how I should optimally mod a gun to give me the best damage output. I am using Gau’s Equalizer as my example but I would think the concepts should work for all.

So, Gau’s drops standard with all 5 damage types (3-4, nano’able up to 5-5) and up to 75 attack strengths of each type.

Ideally I would like to ensure that I can phase, stun and ignite my targets as reliably as possible. I do have crit from items and expertise which I obviously want to max as efficiently as possible but I would like to be applying all sfx’s as reliably as possible.

Should I be focusing on mods that give +% of a damage type? Or look for mods that give +X attack strength? With Gau’s I only have 2 slots so I need to be very picky - my initial intent was to go for X% damage with procs but I would like to know more about how the damage is calculated. Any information that anyone has will be much appreciated. Thank you!!

Edit:

Also, if anyone can tell me how the Thorns damage on items works I would also appreciate that. Thank you guys and Merry Christmas!

thorns hit enemies that hit you, number is the damage that will be dealt, all thorns activate at once and count as performing a melee attack.

applying SFX because is weird - there were several discussion on Discord and things gets both more clear and more convoluted each time. how hard you hit matters, but i don’t think individual element’s damage matters? +x attack str will help, but it’s probably not worth sacrificing CCM+CDB for it. also, damage increases are split into groups - damages in one group are additive to each other, but groups are multiplicative, and IIRC +% all damage is in the same group as critical damage. i think it’s better to use crits once you have over like ~20% crit chance?