I'm a bit torn here. I've mained Garen since I started in season 1, and I plan to keep playing him in the future. The juggernaut update really didn't change much for me. He has the same playstyle as before, only now he's a little weaker early on to certain counters to balance a better late game, and it's easier to snowball with him if you play your cards right. Some of the changes felt unnecessary, and some still don't feel that great in game, but overall I think it was a positive update for him.
I do have some disagreement with what you're saying, though. For starters a good Garen player will be using his ult on a priority target, regardless of whether they are the villain or not. Sure the large true damage number looks bad, but for most targets the same exact result would have happened if it was magic damage instead.
Secondly your CDR is off. Not only are those two items equal to 30%, but even if they were 40% Garen's ult would still have a 48 second cooldown at max rank. I don't know what kind of team fights you're having, but I don't remember the last time I had one last nearly a full minute.
Third no one should consider Garen's passive sustain. It's fully out of combat. "Sustain" on nearly every other champion helps them keep going during a fight, and so I really don't think Garen's passive counts. Yes, it heals him, but it's fundamentally different compared to almost all other self healing abilities in the game. On top of that his passive basically doesn't exist before level 11. Seriously, there are champions in the game with more base health regen than Garen has with his early passive. You can't just focus on his level 16+ passive, you have to look at it all. His great late game healing is balanced by it's crappy early game.
And finally Garen is actually less tanky than he was before the juggernaut update. You can't say he was fine before and also say he is "unkillable" now. He has some nice, tanky itemization options now that he didn't have before, but he lost his sizable percentage resistance boost in the process.