A tank that uses HP as a resource is, in mu opinion, not a good way to build a tank, especially if you start adding a DPS increase based on how low the tank is.

Even taking Torment's Unleashed into consideration, 5% is not much HP and a 3 second invuln is way too short to react to. Irregardless of that, having an invuln that is essentially Superbolide, but on a 2 minute cooldown is ridiculous, especially if you take into account the fact that you clearly intend for the DRK to heal themselves, so the healing is going to be quite big (also, FYI, 5% of 80,000 is 4000 HP for a tank, I know the joke is auto attacks tickle, but even then they could tickle you to death).

Even then, you have a trait that rewards low HP, yet your basic combo heals you, which is counter to what you want the job to achieve.

Also, adding in a party wide damage boost would inevitably make it a top pick, there is a reason why none of the tanks have that ability after all (the same goes for why none of the tanks can permanently decrease incoming damage).

Overall, it suffers from what I see in most 'reworks/reimaginings', there has not been any thought put into how the kit potentially interacts in a party setting or how the job would actually feel like to play within FFXIVs combat system. I will give the same advice I do to anyone, think critically about your ideas. You have this idea, but how would it work in the game, would it make things unbalanced, would it even feel good to play etc.