after leveling Paladin ... Dark Knight just feels really squishy

[TouchandFeel]

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Popping your defensives during a pull is actually very inefficient and tends to result in wasting most of the up-time on them. With all defensive cooldowns that are gated by a recast timer you want to maximize their effectiveness by timing them with the greatest periods of incoming damage. In a mass or long pull that tends to be approximately 2 or so seconds after you stop when all the enemies, including the stragglers, have caught up to you and are now attacking you.

I personally tend to queue up Salted Earth so that it is ready to be placed as I am getting near to stopping, place it where I will be tanking, spend approx. 1 or so seconds making sure positioning is good. At this point the enemies that were chasing me should be catching up so I pop whatever defensive I will be using for this pull plus Blood Price, throw out my initial burst AoE enmity to build my cushion and then start in on my AoE rotation.

Really one of the best "defensive" abilities to use during the actual process of pulling is Sprint. If you put enough distance between yourself and the mobs chasing you by outpacing them slightly, they can't hit you and you take little to no damage from them. Just keep in mind where the rest of your party is and ask them to use sprint as well if they keep falling too far behind.

While you may mitigate a little bit of fluff damage by popping your weaker defensives during a pull, as you said, you should be using those weaker ones in your basic defensive rotation. As has been stressed before, while using these weaker defensives by themselves may provide little defense, stacking them together can pretty much make them just as effective as something like Rampart.
Anticipation is on a 60s cooldown and so can be used almost , if not every large pull. It's availability makes it good for popping after your main defensive has run out and you want a little more protection until the mobs have been thinned enough to where their damage is negligible.
With Convalescence and Awareness on 120s, they can then be planned for and stacked with every other use of Anticipation to provide a good amount of protection.

Specific to DRK, DA+Abyssal Drain is a must for survivability for big pulls pre-70. Also the DA effect for Dark Passenger does great damage, great enmity and has a blind effect. It is an amazing ability to use when you stop to take on the mobs of a large pull, it has a bit of everything you want built into it; offense, burst enmity and defense. Sole Survivor needs to pretty much be used as part of your defensive rotation as well. Getting that bit of HP and MP boost in the middle of a fight helps a lot. Lastly, to make sure you have enough MP for the next pull, when you get down to the inevitable last couple of mobs in a pack, just spam your Soul Eater combo or just Hard Slash -> Syphon Strike if really low. This is the point in a large pull where your damage contribution to the group is going to matter the least since the DPS are just going to kill them pretty quickly, so it's better to use that time to get yourself ready for the next pull by preparing your resources.

New DRK players really just need to actually learn what the jobs' abilities are and what they do. If players are only relying on the one-button-press defensives like Rampart, Shadow Wall, etc. , then they are not paying enough attention to their toolkit.

Honestly, the biggest problem with DRKs mass pulling while not having TBN is that they don't have the necessary synergy of TBN and Quietus to fuel their basic AoE resource gain which is kind of necessary for taking on large groups.

Overall the best thing any tank playing any of the jobs in any dungeon content can do to increase their survivability is to pay attention to and learn the dungeons so that they learn the pulls and what defensive cooldowns they can pair with them.
Learn and plan ahead and you should do fine, only play reactively and you're going to have a bad time.