Should I continue my DK?

Nah, no particular order. Paladins are the best "tanks" period, though. I don't think Shadowers and Dual Blades do have any party buffs (except Haste). It's been a while since I played either of them. "Apparently" was put next to Battle Mage because I've only been told this and haven't experienced it myself. Honestly, I had no idea they had any kind of defensive capabilities until I was told this a few days ago. lol

Since you have a Phantom, I'd suggest going with it. It has decent damage, high avoidability, and access to all of the good Adventurer party buffs (and most attacks). Plus, I found it a lot of fun to play after 4th job, since it is very flexible.

So phantoms don't die a lot at tough bosses right?
They're still good after the tempest patch nerf?