Buffing or nerfing Garen seems to draw a bit of contention. A meaningful buff to Garen, in my opinion, would be to speed up the animations on his Decisive Strike, and Demacian Justice would be really nice. I never ran any scientific tests but, compared to other champions of similar abilities, Garen's seem rather slow and easy to interrupt.
Garen's Q may be overall the same amount of time as his usual autoattacks, however, since the damage is back loaded it gives enemies a wide window to counter/avoid it. As for Garen's R, well, it's just straight up slow. While other champions are able to jump in and cast their ultimates easily for a nice pick, Garen can't really do the same. This isn't to say that he should be able to assassinate somebody but, considering how easy he is to kite already, give him a break, please!
Far too often have I lost potential kills to Flashes just straight up breaking the attacks mid animation; and sometimes to the nicely timed CC which other executes seem to ignore.
I personally think that these minor tweaks would allow for higher ranked Garen players to get something impactful while not really affecting lower ranks. Once again, it's not like I did any intensive studying. These are just my opinions formed after many games using Garen. Yes, I am biased in favor of Garen.