I don't think I would go with disengage allowing you to then teleport using your speed. That is way too much cost.
And I would specifically word it so that is can bypass anything that misty step bypasses, including wording it so that you can teleport a number of feet equal to your normal speed. No way should grappling stop you from using it.
I mean there just isn't any reason to try to grind it down into something that no one will want lol just let the cool thing be cool.
I'm fine with a bonus action teleport that doesn't use your movement, or a teleport that uses your movement, but not one that costs both unless it is supposed to be a minor part of the character.
It is, definitely, a cost.
Too much of one...
wibble wobbles hand
It does seem too high for what you want, though, which is totally understandable.
Personally I like it because the Cunning Action improvement of any Rogue Archetype is, essentially, a ribbon feature alongside their main option. Like how Masterminds get to Help as a bonus action -and- a mess of functionality. Or the Thief gets Fast Hands -and- Second Story Work. Or the Arcane Trickster gets Mage Hand Legerdemain -and- Spellcasting.
It also functions as a sturdy base for later improvements. Like allowing you to dash -and- teleport as a bonus action. Though you're right, in my opinion, about teleporting out of grapples and restraints.
Plus it means you could break up your movement as a series of teleports. Teleport away from the fighter and over to the mage. Sneak Attack. Teleport behind your fighter. And as you extend your movement with magic items and spells... Or feats...
But mostly: It doesn't grant additional movement like a half-dash plus disengage option. By manipulating your movement, rather than just adding to it. Which means you'll sometimes still prefer to take the Dash as a bonus action rather than teleporting based on the distance you need to clear.
Have to wonder what the other subclass ability at 3rd level would be, though. I think the most obvious answer is to make your first attack after a teleport be at advantage, since that fits narratively and offsets the use of bonus action for not-off-handing...
But that also feels kinda easy. Also doesn't really "Help" much when you've already got advantage and thus sneak-attack functionality. Maybe something like....
Phasing
As a bonus action you convert your movement speed into a Phasing speed until the start of your next turn. While phasing you can teleport to an unoccupied square that you can see. Subtract the distance traveled from your remaining Phasing speed. So long as you have Phasing speed remaining, you can continue to teleport as part of your movement.
While Phasing your are immune to the grappled and restrained conditions.
Phaseblade
If you miss an attack that would qualify as a sneak attack while Phasing, you may select another creature that is within your remaining phasing speed of your target and reroll the attack against your new target. On a successful hit, this attack becomes a sneak attack.
Subtract that distance from your phasing speed.
And hey. Look. Even works on Ranged Attacks. Throw a dagger at a guy, miss, teleport the flying dagger to your target's buddy!
But, y'know, YMMV. I'd definitely suggest talking to your DM/Players about what your table's threshhold of "Too Much" is.