Tahm Kenchs Kit is inherently broken and always will be

No amount of numbers adjustments will ever fix this. He's either going to be broken OP or trash tier, never in between. His kit doesn't allow for anything in between because of its nature. His shield gives him the ability to absurdly tank so much damage, literally the only way to kill him is by bursting him (good luck he's full tank). He basically absorbs all the damage first, then pops his shield when his HP is low, then backs out around the time his Shields HP is around half. You're basically killing to lifebars at this point, and he's already stacking resistances as it is.

You also have to consider how powerful his ult is in combination with his devour and the little downtime it has. It's cooldown alone at Max rank is ten seconds, meaning with 20-40% CDR, he's sitting on a 6-8 second CD on it. I understand he has to apply 3 stacks before he can eat a enemy but he already has tools to get to a backline WHILE BRINGING someone with him, so applying those stacks isnt a problem. Even if he doesn't get his target killed after spitting them out, he's disabled them and positioned them out if the fight (and done a good chunk of their Max hp in % magic damage). Every draw back that he's supposed to have, he has clear cut answers for. Not to mention he can constantly safe and protect allies too, which makes it extremely frustrating when you've landed a clutch CC spell on a carry only for him to say "nope."

He's also impossible to duel for a good chunk of the roster because you cant kill him. He just outlasts you and burns you down with Max %HP damage that's already really high, so he doesn't even need to itemize spell pen. Just full CDR tank.

There's no way you can balance this kit. Increasing his cooldowns just makes him shit. Reducing his shields strength by any significant margin just makes him shit because he's not able to do his job then (absorb a fuck ton of damage and burn people fown and disrupt) and that's only after he's ulted into a backline.

I'm really perplexed at how a design like this made it to live, he's going to be pick/ban in competitive play instantly. I get making interesting mechanics that bring more to the game but tahms entire design feels like its just full of flashy abilities because why not.