I have to disagree here.
A "young Garak" series would have two big disadvantages.
First of all, we would have that constant messing up of the character's background story. The series would most likely screw up the excellent written background story which Andrew J. Robinson himself created in the book "A Stitch In Time".
I seem to remember A Stitch in Time leaving lots of unfilled time in Garak's backstory. It's been a long time since I read it, so I may be wrong, but I don't think the idea that he might have infiltrated the Federation at some point automatically contradicts the novel.
The second big disadvantage is that we wouldn't have the benefit of Andrew J. Robinson as Garak.
Yeah, but there again we might get the benefit of another brilliant actor. That's not so much a "disadvantage" as it is a creative decision with both pros and cons.
And having Garak as a spy in the Federation would not only screw up the background story in the bokk but we also had to see Garak pretending to be a human with a human face most of the time which wouldn't be the same as seein the "real Garak".
I mean, that's a feature, not a bug. There is a point where a creator has to acknowledge that it's questionable whether or not a mass audience of millions of people will be able to connect to a primary character wearing a rubber face as well as they would to a main character where we can see the actor's face.