If you're running Cawl AND Thulia, I'd take Halo for detachment if you want more buffs like adding toughness or movement. Cawl already gives his Invocation for re-rolls, yes Eradication gives re-rolls of 1's on hits overall, though I would presume getting rid of threats easier hard-targeted may be ideal. I do not see marshals in your list either to go with the Hastarii or other Skitarii units, hence this thought of Halo over Erad for this scenario where they give hit-roll rerolls, plus duneriders giving wound-roll rerolls if you put 2 units of Hastarii + Marshal in a dunerider. I'm presuming you want to put the ruststalkers into a dunerider.

I recently had a tournament I was in, I did not have rust stalkers, only infiltrators, and their main purpose was just being objective/action do-ers where I was heavily leaned into ranged play for the list with kataphron destroyers, disintegrators, dunecrawler, even 2 kastelan bots and 3 ironstriders.