Just for the sake of clarity Garen is not good to one trick, he takes way too much effort to be as useful as other champions that do the same job. That said if someone did put the effort in he can be made to be good. I definitely wouldn't recommend him to someone who just wants to one trick their way through the ranks, but he's not a waste of time at high elo if you just want to play Garen yet also want to try climbing.
Garen is currently considered (by some) to be the best champion in the game to play when it comes to learning the game. He scales so hard with macro play and punishes you for macro level mistakes, all while mechanically being faceroll easy to play. Unless you're a diehard Garen fan the entire reason to play him right now is to learn what you can and apply it to the rest of your champion pool.
I don't mean that one trick Garens are bad just that you hit the skill-cap relatively fast so one tricking doesn't do much
That depends on what kind of skill you're talking about. Garen is from the old days when people would say League has two aspects you have to learn, mechanical capability and general game knowledge. This is better summarized as micro and macro play, respectively, and isn't really talked about anymore with how many mechanic heavy champions there are compared to game knowledge champs. Garen is clearly from the latter group, and so macro level play is the skill he scales off of. If you hit the macro skill cap then that is honestly impressive, but most people only consider mechanical skill and leave it at that.
To put it simply a Garen main won't be making any Faker level flashy plays, because he physically can't, but he can rotate and split push and catch your team off guard by playing the map at a level that is generally a few steps higher than other people of the same skill rating. I truly hope you get to see a good Garen player some day (please don't take me as an example, I'm just a lowly scrub), because while it's not an impressive sight it is worth seeing.