You are right... I think there is still some adjustment needed to make damage mitigation a bit more impactful when leveling up your good TPI Illuvial. It dose not seems to scale as good as the Damage modifiers. which makes "tank heavy" teams a bit underwhelming compare to the "super DPS carries". But lets see what the next patch brings, I dont think Leviathan has been fully balanced yet... we also get new "improved" TPI/level modifiers so we need to see how they impact the different comps.
You are absolutely right about the Invulnerable drone augment, it is too powerful... could change it to something like 4.5sec 50% damage reduction instead. it would still be good but not as OP as it currently is.
Some Augments are better with "stat scaling" than others (those that give "boosts" based on %), but dont think anything is "broken" and think we need to see how the next patch works first.