Truthfully is sounds like you've got a pretty good handle on how each plays already. Though you're missing out of Hurricane as one of stam sorcs defining skills. It's a good damage AOE when it ramps up and the minor expedition full time is awesome, because it's the effect of a cast rather than a passive like on Concealed Weapon it carries over when you swap bars.
But I'd suggest moving past thinking of a stam/mag divide within the classes. The reality of the game now, especially for PvP is to go hybrid with high health, high wd/sd, high recovery and low maximum numbers for both stat pools. Then you just mix and match skills based on which is the best morph for your build without worrying about what stat it draws from. Probably more than half of Cyrodill is running a magic weapon on one bar and a stam weapon on the other.

Edit: added more detailed observations about each of the classes.

NB-
What you said about the squishiness is true for sure. But the more experience you get with timing dodge rolls and blocks the less squishy you'll feel. And speed is definitely a great way to stay alive. My NB uses mostly stam damage abilities but I run Concealed Weapon, not just for the 10% damage boost but because of the permanent minor expedition. Then I use the Ring of the Wild Hunt for another big speed boost while in combat, and if things get sketchy I hit a dodge roll on my Bow bar for Major Expidition, flip back to the front that has Concealed and Cloak, sprint out of detection range then cloak and change direction. Most people just can't fathom how quickly you're still moving while cloaked so turning 90 or even 180 degrees off the last direction they saw you going in and moving that way for just a couple of seconds totally throws them off your trail.

Sorc-
Is all about movement. Streak and Hurricane are the signature class skills. The streak fatigue isn't a really big deal in practice. Usually in a fight you'll streak twice or three times in a row at most then stop to fight or heal/buff and 4 seconds will pass which will reset the ramping cost. Haunting Curse might not hit as hard as Shalks or Blastbones but it's a million times easier to use. There's a really good player that always seems to have an uber build with great defense and damage that was absolutely destroying people the other day on one. Applying Elemental Susceptibility with a staff then just keeping distance and pestering with consistent ranged pressure using Bound Armaments and Crystal weapon with Overload and LA's; then timing a streak stun just before Curse going off while people were at half health to give themselves a window to finish the victim. It was really impressive watching them zip around lighting people up one after another.

DK-
If you want burst you gotta use the whip, with three stacks of seething fury it hits like an ultimate. But for the most part you're right about the mobility. If a Sorc or fast NB doesn't want to fight you there's not much you can do, you need people to be willing to stand toe to toe so you can dot them up then get your seething stacks so you can leap and whip to finish. Anyone with a cleanse slotted or wearing Mara's makes a tough opponent. DK's are tanky and have great self healing so they are really tough themselves. You just need to get into situations where people want to fight you and have to get up close to do it, thus why you see so many DK's running around resource towers and rock formations near keeps.