Traditionally DRK turns pain into power. That's why in some iterations of the job they sacrifice HP to do damage to an opponent. In other iterations, they drain life and MP from opponents with sword skills or apply status debuffs with certain attacks. It's why DRK was a very odd choice for a tank job since the majority of what they are about is being a DPS more than being a tank.
Also, because of this theme DRK is the only tank coming out of Shadowbringers that did not get additional healing in their kit to balance out the lower defense of tanks in general this expansion. They effectively "work" at end game because self healing doesn't have as much meaning when attacks are more binary in nature. In fact, Shields are better at end game since it lets someone to go past 100% hp and survive things they have no business surviving, like some tank busters.
Truth is Tank design in general is kind of broken right now compared to other MMOs. Tanks in other MMOs have higher defense and your hope is that the healer keeps you alive instead of letting your hp drop to 5%, they start healing, and then you just fall over. In this MMO right now, you just run through a dungeon like a crazy person and flip the healer the bird if you're a warrior, because you'll just pop defiance and self heal your way out of everything while rotating in holmgang if the group gets cheeky, because all the tanks except DRK are self healers. Then you get to end game and stuff hits so hard that you suddenly have to use mits that you didn't know existed. Like p3s has stuff that one shots 100% HP tanks if they don't blow at least one mit and a healer or DPS used something as well.
Like if that is the direction the FFXIV team wants to go than DRK needs a complete rework to even hope to get there. And if they do that they likely are going to make someone angry on the forums given how many people have complained about samurai simply losing Kaiten. At least the DPS jobs have only one purpose to deal with: Tanks have to both take blows well and do damage.
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So on the constructive part of this whole thing...
1) I believe that TBN does need to have some kind of self heal component instead of being a potential refresh on darkside if your shield breaks. If they do that then it pretty much goes into full cooldown status and no longer uses MP, which is used mostly to determine how many times someone can use Edge of Shadow / Flood of Darkness.
2) Probably controversial but they need to get rid of Blood spiller / Quietus. For one they were a plague on the leveling experience through the entirety of shadowbringers because before level 68 you didn't even have delirium, and you couldn't gain blood gauge off aoe combo, only the souleater combo. They also are just potency increased GCDs with no benefits otherwise. The job has a huge number of OGCDs at 90 and it just does not need an alternative rotation path to be interesting. You're going to be spending all the time managing...
1. Salted Earth WITH salted darkness
2. Shadowbringer
3. Carve and Split
4. Edge of Shadow
5. Plunge (because gap closers need potencies...)
6. Blood Weapon (which speeds up Edge of Shadow)
3) There should be some kind of healing on Carve and Split so it at least matches up with Abyssal drain. However, the problem is capping on healing and the fact that the game is designed around having things either be damage or survival with tanks. An attack that heals is still a damage ability primarily, so maybe giving them a skill that lets them self heal from attacking would be better than giving carve and split a healing potency.