[2.0] Hybrid SRS - The Not-So Glass Cannon. 5k+ Life AND 5k+ ES

(all calculations below assuming level 20 gems) From Modifiers, the multiples come after damage conversion, so for a Raging Spirit in a Hatred aura, it is doing 50% Physical, 50% Fire, and 36% Cold (percentages are off of the base Spirit) Assuming you have all Minion Damage nodes, you have 105% increased minion damage, for a basic damage of 278% of base roll.

Multistrike gives 94% more attack speed for 30% less damage, which is all multiplicative, for roughly a 36% damage increase, (1+0.94)*0.7 for a damage factor of 377% of base roll.
Melee Splash and Melee Physical Damage gives 57% and 49% more physical damage, but since it is after conversion, you only go from 269% (sum of all damage types, including the node increases) to 337% and 329% of the base roll.
Melee Damage on Full Life gives the same bonus as Melee Physical damage, and is normally safe as mobs do not attack spirits, unless you are against a mob that does AOE damage.
Minion Damage gives 49% more damage (no type limitations) and so would result in a damage of 415% of base. (Pre 2.0, Minion Damage gave increased damage, which is added to the 105% for a damage of 345%.)
Empower gives +40 base damage at level 20 and 21, +50 at level 22 and 23, etc, which translates to a ~10% exponential base damage per level of empower. Level 2 Empower is weaker than Melee Splash and Melee Physical Damage. Level 3 Empower is weaker than Added Fire Damage. Level 4+ Empower is better than everything.
Added Fire Damage occurs during type conversion, and adds 44% of the base roll as extra Fire damage, which turns out to be 369% of base roll.
Added Cold Damage, Added Lightning Damage, and Added Chaos Damage are additive, before multiplication and conversion. Of these, Added Chaos Damage is the best, at ~400% base roll on average (the added rolls are actually separate from the base rolls, so they have more variability, this is just an average.)
Synergies - Since Melee Splash and Melee Physical Damage all operate on Physical damage, they do multiply rather well. Comparing those two to Added Chaos Damage - Added Fire Damage still loses out, to the tune of ~20%, the advantage being that Physical Damage typically is mitigated less than elemental damage. (Especially since modifiers like added Fire Penetration or increased Elemental Damage from passives do not apply to minions.) An alternative may be Melee Splash and Added Chaos Damage. The loss here is only about 10%, in return for splashing and better chances of getting through against higher level mobs.

+2 Minions, +2 Melee, or +2 Fire (for item mods?) - The three Melee skills are, of course Melee, but the Raging Spirit itself is not. +2 Minions vs +2 Fire comes down to Added Fire Damage vs Minion Damage. Since Minion Damage appies to all converted damage, +2 Minions is better.

In order of preference:

Spell Echo to give more Spirits
Minion Damage
Added Chaos Damage
Multistrike
Empower depending on level, but since it takes so long to level, it's probably the last one on your 6 link anyways
(if you don't have a leveled Empower)

Added Fire Damage or Melee Splash (Melee Splash may turn out better due to mitigation on the mobs)