I'm sorry but it Doesn't need more self sustain.
Would it be nice for some more? yes of course some extra sustain would be nice but it doesn't need more self sustain in dungeons, I feel very safe on dark knight if I'm actually using my mitigations well.

But being cautious about self sustain, Warrior for example does this in a unhealthy way where every defensive cooldown practically has a massive form of self healing. It's fine to have some sustain options but having sustain tied to too many abilities has been seen to take away gameplay from healers.

to make DRK slightly better (in dungeons in particular) Personally I'd add a small sustain aspect to Oblation, Give it a 300 heal per hit you do for oblations duration, But this shouldn't be per enemy in AOE or it would be another case of blood whetting's absurd sustain, Dark Mind should block physical damage by 10%, Dark missionary (heart of light for this sake aswell), should just be for all damage types.

Keep in mind sustain is fine, but the amount of sustain can be too much which is the case of Warrior, I think that job needs to be brought down a bit, or at the very least deal the lowest damage considering its already got the shortest invul making it the king of invuls.