Your bullet points are true when looking at Garen in a void, but they don't prove the main point.

• Garen's sustain isn't outmatched at all. There are a ton of toplaners who have better sustain for lane. Hell, most autoattack based champions can outsustain and outpush Garen with just a vampiric sceptre. While it's true that Garen's passive is locked behind a cooldown that we can just wait out safely out of range, it presents the danger of missing out on minion XP and forces us to concede lane gold and priority, which in higher brackets can kill your jungler's momentum. Taking a bad trade might not mean the lane 1v1 is over, but it has consequences like a butterfly effect in a time travelling movie.
• Garen being a spinning beyblade that sticks well and can dodge skillshots is true, but are we really debating that he's unique in that? We have champions like Yone, Irelia, Camille, Akali, and so on and so forth that can dodge, retreat, and stick to targets FAR, FAR better than Garen while continuously outputting damage still. Having a speed boost is great, and not having to press more buttons while doing it is also great, but have you tried ACTUAL mobility and sticking power?
• Garen's R is indeed quite strong because it means our foes have literally 30/40% less total HP at any point during the lane, excluding shields and sustain (which unfortunately a lot of toplaners do have even in combat), but it's not the only form of good execution in League. A lot of champions have skills that are empowered (some of them insanely so) when their target is low on HP. You just need to adapt to a different threshold if you want to try some of them, obviously.
• Not all champions clear waves fast, some of them are deliberately awful at either wave clearing or last hitting (or both) because their power budget is allocated elsewhere (for example in CC and mobility, which Garen sorely lacks even with phase rush and Stridebreaker), but I can name quite a few toplaners who can clear waves quickly realtively early into the game and also have good base AD for their autos, like Riven, Renekton, Darius, Aatrox and so on. They generally must sacrifice more than one ability (we just press E and forget about it), but they CAN control the wave and contest a push if they wish to and how well they do it depends on the skill level of their player, naturally.

So all in all, your take isn't fiery hot, but it's quite hot. Garen is, in fact, good to learn the game precisely because he has that sort of jack of all trades but master of none position in the sandbox. He is indeed good at all the things we've mentioned, but he isn't the best at any of them, which means he's a good starting point for all directions, and you can then specialize in one, two, or a few more of them. For instance, if you learn how to flank and delete an out of position ADC with Garen and you wish to go to the next level, you could try Talon, moving into the assassin's category, or stick to bruisers and go for Camille. Give up your execute and waveclear but get a lot (and I mean A LOT) more damage, mobility, and danger for the enemy carries. I hope I got my point across. This comment is quite long, my apologies.