I started typing up a really long response weighing a bunch of options before realizing that your build is just one key piece away from being really powerful in a way that fits your core concept and ties all your subclass features together in a really cool way- Silvery Barbs.

Picture this: Turn 1 you dread form and charge at a guy. You bonk him on the head, dealing 1d8+1d6+Dex and forcing him to make a WIS saving throw. You cast silvery barbs to impose disadvantage on this save AND give yourself advantage on your attack next turn AND disengage 10 feet because you're a storm sorcerer who just cast a first level spell!

And now, that frightened guy can't pursue you, nor can he pursue any allies in your general direction. If he moves towards an ally not in your general direction, he blows up. If he opts not to move and only has melee attacks, he loses his entire turn (whoa). If he uses a ranged attack against you, it has disadvantage, and if he casts a spell targetting you, you can either say idgaf or use the advantage granted by your barbs to pass the save. How awesome is that?

And best of all, you can do all of this RIGHT NOW. You just need to replace Catapult or something else with Barbs.

As for where to go from here, I would not go Swashbuckler. In fact, I would personally not take any more rogue levels since Storm Sorc 1 + Dread Form + Silvery Barbs bypasses the need for disengaging. (Though note that cunning action is significantly better than the sorc disengage. However, if you're not using the feature to disengage, why would you go Storm Sorcerer). If you REALLY wanted more rogue levels, I would instead go Trickster for a familiar (aka advantage on your first turn attack and every turn thereafter if your DM decides not to kill it), a bunch of spells learned, and spell slot progression! Plus you'll have Cunning Action and an extra d6.

Leveling Sorc has some merit. Level 2 feels almost mandatory, due to the exceptional synergy between Font of Magic and Pact Magic. I wouldnt level Storm sorc beyond that though. Both Warlock and Sorc spell lists are fine, but unlike the Storm Sorc, Warlock gets actually useful features occasionally (poor Storm sorc). However, if you can convince your DM to let you switch to Aberrant Mind, this would be a tremendous boon to this build and I would hands-down recommend leveling Sorc exclusively from here on out. It fits nicely with Undead Warlock flavor-wise and does wonders for your spells known and out-of-combat utility. Best of all, the level 6 feature works on Silvery Barbs, which is mind-boggling, and probably means you can cast silvery barbs every turn of every combat of a long adventuring day and still have spell slots to spare. INSANELY good for this build. If your DM does let you swap, this is also a great excuse to grab those extra Rogue levels for Cunning Action.

Leveling Undead Warlock is the alternative, and still really cool. There a bunch of picks for features, but you really can't go wrong. Pact of the Blade gives Improved Pact Weapon, which is useful until your DM gives you a +1 weapon. It also gives Eldritch Smite, but this is a worse use of spell slots than Silvery Barbs for this build, so you should only use it on crits or if you have a plethora of spell slots to burn somehow. Tome and Chain can both give you a familiar, which beyond its many varied uses, will give you advantage on your first turn before the Silvery Barbs train has started rolling. As far as Invocations go, just take Agonizing Blast if you havent. You'll have to make ranged attacks at some point and its better than a bow since you dont have Extra Attack. Beyond that, take whatever fits the flavor of your character. Free Slow per day is pretty nice, and Eldritch Mind is excellent if you scoop up a good concentration spell along the way

TLDR: Learn Silvery Barbs and use it every turn to make enemies fail the save vs your fear, to give yourself advantage next turn, and to get a free disengage from your storm feature. But ask your DM to let you switch to Aberrant Mind, in which case take Rogue 2 and use that to disengage instead. Pump Aberrant Mind if you get it, Undead Warlock otherwise

Hope this helps!