I would bet that Villainy makes Garen more "oppressive" than his Q damage
And that if you want to address his strength, for low ELO in particular, you should start with this, because Villainy is such a huge comfort mechanic for low level players that, even when the Garen player is playing poorly, he is basically never out of the game. Given that most people who become Villain marked will have at least 2 kills under their belt, and the fact that you are basically not allowed to be near Garen if you're marked or you'll just die, Garen tends to end up getting a lot of shutdown, counter-snowball gold off of what tends to feel like an unfair situation, e.g. Garen was on me for 2 seconds and true-damaged me into the grave.
I'm suggesting this as a long-time Garen player, because Villainy is an overlooked mechanic that keeps me relevant even in games where I'm counter picked or otherwise heavily set back. Reducing its potency might do well to curb his power without making him feel too much worse.