Erasing Strike has a 300% damage effectiveness, so all “added X damage” will be tripled.
In your example the damage from it’s skill tree would be 162 void damage from your weapon, 60 void damage and 102 void damage from your passive tree.
Also the base damage from your weapon (the implicit will increases it’s damage, but is not multiplied by damage effectiveness as far as i know, i could be wrong on that though, but i am pretty sure)
All of the above values will get applied to the base damage of the skill.
(Also note that all multipliers within the skill’s spec tree will also first increase the base damage, THEN comes all the increases from gear and passives)
The base damage will then be further increased by “X% increased melee damage”, “Y% increased void damage” and so on.
Also note that strength will just scale the initial hit damage and attunement will just scale all forms of void damage(most weapons have physical implicit damage, that will not be scale by that), while attunement also scales the damage of the void rifts created by erasing strike.
So it all depends how much void damage you already stacked up compared to the implicit damage of your weapon.
I don’t know which weapon you are using, but i doubt that your weapon deals more than the above calculated 324 void damage^^
So increasing void damage will most likely be the better way for you. But “increased melee damage” is more flexible though, especially when you do not have your BiS weapon yet.
For example if you find a weapon with two different non-void “added melee damage” affixes, that would benefit your more.