Garen is so close to being good, but Riot doesn't seem to know what his actual problems are

tl;dr Garen has a well-defined niche (great peel, late game tankiness/sustain) but suffers from overly high counterplay (mainly that he's very kitable due to an ineffective slow cleanse) and less build options than he's had in the past. Also f*ck that self-slow. However, Riot falsely believes that his problems are related to him being an "easy" champ, while making changes that have exacerbated his kitability and lack of build flexibility. He could be much better following a couple of low-risk/QOL changes: replacing his slow cleanse with a better mechanic (maybe just slow resistance), and giving his armor/mr bonus a flat value and a lower % multiplier (to address the fact that he needs to spend a ton of money to get anything from his W passive), and removing the 20% self-slow through minions.

If you look back to just before Garen's soft rework, the problems I mentioned in the tl;dr are almost nonexistent. He had plenty of other problems, don't get me wrong, I just won't get into those because they were successfully fixed. But the kitability and build flexibility were actually much better because of some forgotten nuances of his old kit.

Firstly, build flexibility: One of the great features of Garen's old kit was that building a giant's belt made you tanky enough for the mid game (old W gave 25 armor/mr with armguard-like stacking; you could get the full 25 at ~level 9). Then you could choose between more tankiness or more damage, and throughout the game W as a whole freed up the slots to build much more damage than the average bruiser. In fact, IE was pretty much a core item, and if you think I'm exaggerating or that I was just a glass-cannoning scrub back then, keep in mind that high-ranked players did this too (Dyrus's core was IE/GA). Admittedly, having that much resistance at level 9 contributed to his general problem of "too good early, not good late" but when they changed W's passive they went too far in the opposite direction. At first it was changed to give 20% of your total resistance, which at least gave him something at level 4, and since that was packaged with tenacity on the active, it was really a net positive. Then his visual rework happened (popularity surge) and lots of people complained about him being perceived as uncounterable (inaccurate overall, but this was related to his excessive tankiness late-game, which I don't necessarily disagree with) so they changed W to 20% of your bonus resistance. This was just an awful change, it still is, and it made it so that you got jack from your W passive until you spent thousands of gold on tank items. You don't get to buy damage anymore outside of black cleaver without huge risk or already being ahead. Garen is the only champion with a resist bonus that gives nothing early. The good news is this is probably an easy fix: if W just gave an appropriate flat value in addition to the percentage, it would open up much better build paths, and since he recently got a huge late-game buff to his passive, you could reduce the % bonus and he would still be just fine late.

Second, kitability: Garen's slow cleanse is almost completely useless. This problem has always existed, but it was much less pronounced pre-rework because your speed boost and cleanse were on separate cooldowns. See, if your opponent is familiar with your kit, he'll hold his slow until after you use Q. This means you have two options: run at him with Q and get slowed immediately, or walk at him while he walks away. This is what I mean by too much counterplay: a smart player with a single slow can negate your Q entirely. With more than one slow you can only hope that your W gives enough tenacity. Slow cleanse needs to be removed from his kit and replaced with something else. I would vote for some form of slow resistance, mainly because I think that would be the least disruptive; Garen doesn't need big changes anymore.

Finally, a little about being easy/low skill ceiling/etc. Riot has said in the past that the reason they can't get Garen to a competitive level is because they would have to make him harder to learn. I strongly disagree. There have been plenty of "easy" champions in competitive play. Garen has just never not had problems. His crappy late-game was replaced with crappy kite resistance and a self-slow, and then his build options were narrowed to "always rush tank". Also, they underestimate Garen's skill ceiling (the whole "good Garens vs. great Garens" thing). You need good game sense to deal with your non-instant gap-closing (flank properly), your R and W timing is very skill-based, your lane phase is almost entirely decided by how well you can abuse his passive, you can make it so that you're spinning while you're CC'd and don't lose any damage (a.k.a. one reason you can sh*t on Jax in lane), and you can micromanage your spin so that you're just out of range of melee attacks in a brawl. Sure, he's got a lower than average skill ceiling, but not that low. Here's one idea I had to raise it: at level 6, your passive can't be removed while W is active. It's just a thought though, my main point is that Garen really isn't this noob-only champ. He has a place in higher level play with his fantastic peel, his late-game sustain, and his ability to help team-burst a tank with ult. All he needs is a few light changes to get rid of his ankle-weights, so to speak.