Hello, i've been playing tahm kench for almost 70 games both ranked and normals, and its very obvious that hes best played as a solo top or jungle instead of support, like he was intended to. I feel that he dominates both top and jungle due to his huge health (thanks shield), high cc and amazing damage. On the other hand in bottom lane hes not so good early on, his cc is unreliable, as it takes a clear path to hit the slow, and a lot of effort to stack a stun, leaving your adc vulnerable.
Lategame, even from support he is about as tanky as other top/jungle tanks, deals more damage than standart top/jungle tanks (malp/shen/mundo/rene) all while having massive amounts of cc. Meaning he can freely be a frontline champion, using his kit offensively, rather than peeling/kiting/saving your squishies. And if he plays jungle/top Tahm is ridiculously strong (especially devourer->full tank build), thats why, in my opinion hes more of a stand-alone front lane disruptor, rather than a great support.
I believe shifting his power to supporting would be beneficial in reducing his mid/late game crazyness as well as increasing the potential in early bot lane.
For this, trade some of damage from his ultimate's on-hit for a heal on his W-devour. Thinking of
-1% damage (=3/5/7% bonus hp) reducing damage by 20 at 4k hp and lv16
3%(+1% for every 750 of tahm's bonus hp) of ally's missing health per second while he is inside tahm (=36% **of missing health** for full duration at ~4k hp and lv5 devour) It might seem like a lot, but consider that you take your ally out of the fight for full 6 seconds (meaning he does no damage during this time).
a change like this would force him to play next to his carries, further reducing his damage (no more yolo chase and kill enemy's carries).