At a certain point - it might just be better to bite the bullet and have Garen do all physical damage. I'm a little leery of this because it does make him ridiculously easy to hard-counter in lane through armor.
Not sure on ultimate scaling at the moment. I'd rather look at Riven/Darius first to get a proper baseline for what these things should look like.
I might be showing my noob, but if his opponents start armor-stacking to counter him, early Last Whisper isn't a terrible option. It'd just be nice for his ult to not get reduced by something that he doesn't/can't really build.