What about creating an ACTUAL Dark Knight?

[Lyth]

...

That's because when you climb Mount Ordeals at around level 15-20 as a Dark Knight, but you're expected to come back down as a level 1 Paladin, and they don't want you to get oneshot by monsters on the way down. Because you gain the first 10 levels or so nearly instantly, you actually have very little stat growth that happens over those levels. There's actually very little difference between a level 1 Paladin and a level 10 one. It's more for storytelling effect.

Either way, no two Final Fantasy games are alike. Shiva is a motorcycle. Ragnarok is a spaceship. The concept of "tanking" doesn't really exist in other games outside of FFXI, so what job fits what role is largely up to your imagination. Blue Mage and Dragoon could have been tanks. Paladin and Dancer could have been healers. Rather than lamenting what could have been, maybe it's worth focusing on this iteration of the game?

If anything, FFXIV has the most interesting spin on the PLD/DRK duality. In FFIV, Cecil became a Dark Knight in pledging service to Baron and its institutional corruption, but found freedom in becoming a Paladin. In FFXIV, Paladins are historically trained as bodyguards who swear oaths in service of the Sultanate. Dark Knights, on the other hand, swear allegiance to no master. Their only duty is to follow their hearts, to protect the weak against injustice, and to punish evil as they see fit. As a classically trained Paladin who switches to becoming a Dark Knight in the Brume of Ishgard, you're essentially following the same path to freedom.

"We dark knights don't care one whit for prestige or pedigree. We are free to follow our hearts─to defend the weak and punish the guilty as we see fit.
The law of the land? The authority of a name? These are tools cowards use to escape harm. We have no need of shields figurative or literal."

[Shurrikhan]

sacrifices their own HP to protect themselves

That makes no sense whatsoever in a tanking scenario. =p

Sacrifice HP. Gain defense. Now the HoT placed on you is no longer overhealing and the following tankbuster won't one-shot you, so long as you're topped off before it.

How does that "make no sense whatsoever" for a tank action? You're literally smoothing out damage intake, exploiting lulls in damage to prevent spikes.