Keep in mind, this is coming from a Riven main, so it doesn't mean much, but I have some ideas that might help balance Garen.
1. Make the cooldown on his Q go down by level instead of having a flat 8 seconds
2. Reduce his base movement speed, he has the Q movement speed buff, also it will make it more difficult for Garen to stick to his target while E is active.
3. Reduce the time Garen has to use the silence after activating Q As Riven, he can activate Q, but if you stun and E away, he can still catch up to you and silence you.
4. Reduce the Q and E bonus AD ratios. The Q AD Ratio does more than his base AD already (140%) PLUS it deals the bonus damage from the ability itself
[PHYSICAL DAMAGE: 30 / 55 / 80 / 105 / 130 (+ 140% AD)]
[PHYSICAL DAMAGE PER SECOND: 20 / 45 / 70 / 95 / 120 (+ 70 / 80 / 90 / 100 / 110% AD)
Thanks for reading my post and drop a comment if you have any other ideas...