I don’t think Thorns-based builds have ever been a thing in any version of Hellgate. It’s a passive mechanic, after all, and so has to be relatively weak. Also beware that while the skill planner description has Aura of Thorns as a flat 60 damage increase per enemy within the Aura at maximum skill level, the in-game description has a 60% damage increase per enemy within the Aura, i.e. the skill boost existing thorns damage from gear.
As for the exact mechanics of thorns damage, I have no idea. Maybe it uses the mainhand weapon for crit chance and other boosts, but frankly, I doubt that.Thorns damage comes mainly from armor, after all. So I find it rather more likely that thorns would have no crit chance, weak or no elemental attack strength, no shield penetration except for what comes from armor, and no good way to boost any of that beyond what the aura provides. In the same vein, I would be surprised if it triggered on-hit procs in weapons and weapon mods.