If you are good at PDT swapping and make a Stun toggle for enmity spikes among a few other things. You can go much farther than this than most know. I've tanked Sinister Reign this way holding hate reliably with sword and board/2hand. Many T1/T2 Escha NM's, apex parties etc. I find myself a very good safety in events where I can grab a straggling mob or hold an add/boss away for some reason. If ***hits the fan I don my tanking booties and try to shovel the *** off our port side.
First thing you need to look into is to get a -100% interruption set via the eschite gear, the founder's hose and impatiens. With this and a good Fastcast set you can never be interrupted with spells of your choosing. I am pretty sure, that before the eschite gear was introduced achieving this on DRK was impossible. When subbing blue mage, ninja, white mage etc. . . it is nice to have certain spells be unstoppable: any enhancing, blue magic, ninjutsu (anything you want really as long as you know you are sacrificing potency) but in many of the useful cases this is irrelevant. Uninterrupted aqua-veil/erase/whatever-I-want changed my life.
Second, I like the steadfast shield. Seems like the best shield, but even when "tanking" I still use two handed weapons mostly. DRK when wielding a scythe basically has infinite MP. If you have good timing you can cast spells through melee strikes, and spells with a longer cast time you can fall on your -interrupt set.
Third, you need an enmity set for aspir, stun, flash (at least) and also some way to toggle enmity sets into applicable spells versus spell potency.
Absorb-TP can be a strong hate spell. Unlike the other absorb spells which produce less CE. I have not yet used this toggle since making it. With the other options I have for hate I just go for the potency.
WHM Currently I can CureIV myself for 900+~ with new gear. I have an enmity toggle for my cureIV too. I don't find myself needing that one as much as the others. Stuns and flashes build up a lot of hate, aspir is a good tool giving about 300~ enmity, Aspir II seems to work the same way. Granted the spells cast in enmity gear are not at their most potent, but we just need them to be cast for hate.
BLU has some really great enmity tools and with a enmity+ set they just keep hate so damn well. Cocoon is great of course but Wild Carrot is not as good as Cure IV and I find myself missing AquaV, Stoneskin and Blink I don't use this sub as often as /whm.
Then. . .
Of course, if the mob doesn't dispel and not immune to Drain with a dark SC I'm a God sitting at 4,000+ hp full time on a bad day, 9999 on a good one. But if those two things don't pan out you can survive a long time. I am not saying you need to have a giant Drain III to tank, I'm saying that when it does work, it is just ridiculous.
Tanking is quite possible with the native HP pool, and even if you could only get a 300~ Drain II/III on the mob, take it. 300HP is pretty sweet. I know jobs that would rob an orphanage blind for another 100hp.
But if you have a good way to switch in to hybrid/offense/turtle sets, and can time your PDT/MEVA with the mobs moves. You'll go very far. Will you be the best tank? No, but it is better than nothing for a lot of things, and in an emergency it works to grab hate so you can get your tank up and not wipe. Honestly, I am probably underselling it.
Also: Bio III -20% attack (Bio II takes the edge off too), Infernal Scythe -25% attack, and G-Wilt -toomuch% attack. These can be thrown into battle to help lower the monster damage and the party recover. Bio overwrites Dia for a reason. Pretty sure attack down is a water based effect, so even if infernal scythe gets resisted the additional effect lands on a lot of things, weird things even, things I never thought it could. . .
Q: What
CAN'T you tank with 6000hp and 25%-50% DT?
A: Pretty much *** nothing.
Beware: you need be on point with your PDT/MDT swaps for mitigation, and have your enmity/interrupt stuff sorted out. Also, when holding hate, and trying to survive on DRK it is intensive that there is a very small margin for error against harder enemies.
In the end, if you cannot switch between using your spells for hate generation/utility or potency on the fly, don't do it.
If you want any more detailed information I would be happy to answer further questions just pm me.