Personally, I'd rather each of the Invuln skills were revised to... less of an invuln and more of a unique mitigation+utility skill.

Consider, when Holmgang ("Going to the Holm (a small island on which duels could be held)" or effectively just "Duel") first came out, it literally just tied the Warrior and its target in place, allowing certain kiting strats (when others had threat on its target) and ignoring knockbacks. The Warrior mog in the Good King Mog fight more or less copies this with a mass draw-in and snare skill. It'd make sense, perhaps, for it to prevent dying from any other targets while causing damage taken from any other sources to build Beast Gauge or the like, but a general death prevention seems a bit out of left field, as fun as it may feel at times.

Living Dead would be mostly fine if it just had a shorter cooldown (or if the others were nerfed to compensate), if it had a further effect during the actual Living Dead (not Walking Dead) component, and if DRK had decent life-steal, at least contextually. Make the skill as a whole more granular, effectively just allowing it to drop you into negative HP without dying instead of an all or nothing healing cost penalty, and give DRK a Blood for Blood trait, increasing its life-steal based on %missing HP (including the amount stored in Living Dead as negative HP) atop some lifesteal revisions across the whole kit. If capped to as much fatal damage absorbed as your maximum HP, it'd effectively allow no more and no less than twice your maximum eHP, but without any of the added healing efficiency; instead it'd tie into synergies with DRK's own kit to make up the difference.

Superbolide is... strange. The "hit just before death to minimize cost" gimmick is fun but, there's little else good, let alone unique, to be said about it. Not sure how I'd change this one yet.

Hallowed Ground is iconic, and therefore can't rightly be removed, but neither does it have to be free. It could, for instance, as easily force a 1-bar Tank LB on use. I'd be fine with Paladin being the only job to retain an actual Invuln. It just needs to have some balancing cost therein that makes it a tactical choice for one's party, rather than a free bonus.