Garen mid is fun and broken
I've been playing Garen mid for a few weeks now, and I honestly think the pick is completely broken. At least in my humble Gold elo.
There aren't really any unplayable matchups (I ban vlad every game). The first few levels can be rough, especially if your opponent knows how to punish your lack of range.
Once you hit level 6, though, you've usually stacked enough W passive resistances to become almost unkillable for most mid laners. At that point, almost nobody can contest your waveclear with E + Tiamat. Kind of like Akshan, I instantly delete the wave and immediately look for a roam.
Roaming is also super easy since you're building full movement speed. Then once the mid game starts, split pushing becomes incredibly straightforward thanks to mobility and tankiness.
I also think the pick scales well. It's honestly one of the most fun picks I've played in a while.