Literally everything about this is incorrect.
Firstly, TK’s aren’t good at defending siege. In fact, they’re probably one of the worst options for defending siege. If someone comes after your siege with BBCs, having TK’s there just means that now you lose even MORE gold than before. You’re far better off having Halbs or Champions there, because the potential losses are lower and they’re either just as effective, or moreso, and they can actually kill things that are sent to kill the siege, rather than strutting around waving their capes impotently.
Secondly, the entire reason TK’s are hard to balance has nothing to do with their stats, it has to do with their function. What do they do? Basically the exact same thing champs or halbs do, just slower. But if you can have Champs or Halbs instead, why would you ever bother? Especially with the new armor upgrade. So you’re stuck trying to compensate for the absurd vulnerability of being slow, except that’s impossible, because as long as something can be easily kited, it’s never going to be as useful as a slightly less powerful unit that is much harder to kite. And that’s not even getting into their comparatively massive vulnerability to conversion.
And finally, I see absolutely no reason why allowing something other than villagers to build JUST walls and towers would be anything NEAR ‘insanity’. Not any more than allowing towers to build unique units, anyway(kreposts), or allowing food to be instantly dropped off from farms(khmer).
Much the opposite; it would actually give TK’s a reason to exist for the first time in their life.