Yes, the slave drivers are resist capped, I made sure of that via PoB since they have odd resists compared to the usual spectre(more lightning, less fire and cold). They even have +5 max resist from Elemental Army. Spirit Offering is not bad but I'm also not sure how much that would help. You'd have to be casting that quite often.
I"m more annoyed with how annoying Spectres are to resummon, and holy fuck did they sound annoying pre 3.8, it's part of the reason I never played summoners before this patch. If they *can* die, you have to assume they are going to at some point despite your best efforts. And it's at that point, being stuck in the middle of a fight with no spectres due to stupid AI, that not being able to easily resummon them feelsbadman.