Do not let this champion fester in top lane. He hits almost every single hallmark of a terrible design.
Tahm Kench is the least interactive lane bully and cannot be punished. Even Pantheon and Renekton give leeway for opponents to handle them. When Tahm is open you can either play a ranged champion or spend the rest of your lane under tower. Tahm missing his Q doesn't matter at all; if you try to fight him you are only doing his job of engaging for him.
Tahm cannot be poked out of lane. Even ranged champions have to concede lane priority in order to let him push and spam Klepto on his fat head. His high regen makes sure he rarely dips low on trades and pokes and all inning him is suicide thanks to his ridiculous shield values. This same shield is why Tahm can set up effortless 100-0 tower dives with his jungler and kill you. Simultaneously, your jungle ganking him is a complete waste of time since he takes so ridiculously long to kill even in the early game.
Tahm is horrible late game. This champion is old Nunu on crack and shoved into top lane; he will make YOU useless and completely hate the game you're playing and then become useless himself. Except not really; Tahm maintains a high win rate in the 60%s for the first 25 minutes of the game that still dips to above 50% after 30. The average game is about this long so in the "late game" that most people actually play into he's still a menace.
If you are going to balance Tahm as a top lane then you need to BALANCE him as a top lane. Some suggestions;
- Passive stack duration nerfed to 4 seconds and a timer is put on the debuff that appears on your champion bar.
- Q slow buffs reverted.
- E shield is capped to a % of Tahm's health until rank 3 of the spell.
- Bonus damage passive put back onto his ultimate.
- Q+W being free reverted
Of course you could just trash him instead and I wouldn't regard that as a loss. Tahm is one of the worst designs put into this game and shouldn't be viable at all.