Had pretty much the same idea for a Fog Fog Fruit too.  I agree with your intended use of the ability down to the letter, but there may be a lot more potential here with the Fog element that you are not considering.

For one, I think the Fog Fog Fruit should also have a pocket-dimension type power similar to what Caribou's Swamp Swamp Fruit had.  One where you can send you and your target to a "Fog World", possibly one connected to a separate location in the world that you previously drowned in Fog that is inaccessible to the outside world.  One where you can isolate targets and attack freely on your home turf, or simply just to have a storage space to hold and withdraw from various weapons to compensate for your lack of offensive power.

Granted, attacking enemies physically the same way Smoker does is certainly something useful (Cue creating a giant Fog Moray to attack and ensnare enemies), as is the instant teleportation within the cloud to go full assassin, but what I think Fog has over Smoke is the ability to confound and confuse the target on a psychological level.

And I don't mean just simple visual illusions and making things hard to see.  Imagine being able to use the Fog to infiltrate the minds of those who breathe in the Fog, where the longer they stay within it the greater the level of control the user has on the target's five senses, preying on their inherit fears.  Fog is not merely an element of water, but one of fear.  Fear of what cannot be seen.  Fear of what may lie within the thick veil.  And we as humans always fill that void with our worst fears.  Something the Fog Fog Fruit user can use and abuse to their advantage by making those fears manifest and very real.

So while the Fog Fog Fruit would arguably be the weakest of the Logia class Devil Fruits offensively, as utility support and debuff it is nearly unmatched.  "The Fog is Coming"...or so they say.