Specter is an absolute mess right now, you wanted to make a single target support class but ended up making just another cookie cutter condi DPS that is not even good at that.
First of all, the single target idea is just bad, neither the game nor the content in it is made for a single target healer, only place where that would work is in 2v2 sPvP, and that is a temporary game mode. Guild Wars 2 is an MMORPG, Massive Multiplayer Online Role Playing Game, so by that single definition is obvious WHY a single target support does not work, we are supposed to play along with many players! the social aspect of the game is at the absolute core of the experience, the community is amazing, so why would you make a class that goes AGAINST this type of gameplay?
there is a VERY good post detailing why it is such a bad idea, so instead of repeating it i will just link to it:
If you ask me a good solution to this would be to make the abilities ground targeted with a narrow AoE range, so could be something like 300 units of effect and 1200 range, that would make specter work while still giving a limited area that would require skill to play.
Second. you want to make thieves supports then LET THIEVES SUPPORT alacrity alone is not enough to support a group, or to fill a support role especially on a class as clunky as Specter currently is, give us the ability to provide what the party needs, at least give us good options to give AND MAINTAIN Might and Quickness at the very least, the rest of the stuff, fine, other classes can provide that, but at least let us have good ways to give what a group needs and please for the love of anything that is good forget the single target idea. IT. DOESN'T. WORK.
Third, i know that you want to give us a reason to use shroud, then the idea is not forcing us to stay on it, give us a REASON to stay on it instead, here's what i propose: make it so that staying on shroud allows us to give more buffs to the party/squad, give buff options on the shroud abilities, that way we have a good encouragement to use it and not immediately consume it for healing.
Fourth, speaking of shadow shroud, give us better options to gain shadow force, another reason why nobody goes into shroud mode is that there is not enough ways to gain shadow force, so the best option is to just go in and then back out, if you gave us good options to gain shadow force along with good incentives to maintain the shroud (healing and buffs) people would use it more.
Furthermore, if you want to make specter a focused support class, a class that focuses on one target to support, here is what i propose: give us a main support target, this target would get stronger versions of the buffs while the rest of the group would also get them just on a slightly shorter duration, let's say for example, your main target could get 8 seconds of alacrity, might x3, and quickness while the rest of the group gets 5 seconds of the same buffs.
and for the last time, so that it is CLEAR:
SINGLE. TARGET. SUPPORT. DOES. NOT. WORK.