Why is DRK so squishy and how do I stop it?

[Februs]

in theory, the only difference between a DRK and a PLD is blocking, and with low level scaling that doesn't even happen very much.

I wouldn't say that this is the only difference between them. Pld's passive block mitigation is a more obvious distinction between them, but there's also a pretty subtle (but substantial) difference in how their CD's are used, particularly in how they are timed. A lot of people see the Drk skill sets and say, "oh, it's kind of like Pld," and they're not wrong. It is "kind of" like Pld, but if someone tries to play them the same way then I can understand why they'd feel squishier.