Hi 43rd. I dont want to down you, or start another Tamiya VS all Fight, but just some tips from a Tamiya King Tiger owner to a Tamiya king tiger owner:
That idler kit that does not inspire confidence is a piece of shit, but you have a couple of alternatives, you can install the Daryl Turner system (pricey, but David Turner stuff is AWESOME). That is rock solid and works with springs too. I have not seen it in real life, but people speaks marvels about it. The second alternative are the Henntek track tensioners. I have installed one and its bulletproof too, but it has no springs. The third option is to throw away those small springs and replace them with putty. (Not tested by me, but I think Tom Hugil did it)
As you have mentioned, the plastic is thin and fragile, but I am afraid the aluminium lower hull is not much better. A gearbox brace can do marvels on your tank.
The third fragility problem you will find, are the tracks. As you can see them are the "Open pin" mode. That means in the tracks you can see the entire pin, and one link is linked to the other just with a couple of milimeters of plastic. As you can imagine it does not last long, and a lot of people breaks them on the first run.
I considered buying metal tracks, but they are really expensive (still can digest how some pot metal tracks are more expensive than my 1000cc Honda motorbike chain, lol) and also it seems that with the metal tracks is mandatory to do the mods I told you (idler and gearbox brace) and also to add some more mods to prevent hull flex. But if you want metal tracks you need to buy the impact ones, or the kenny kong. Mato, Taigen and Heng long ones are not compatible. It seems the "Matorro" ones are compatible too, but not sure about it.
Im sorry, but that's what it is...
Keep on going with your build, and try to install working lights on the tank.

I put a lot of extras on it: Headlight (with warm white led!, not xenon white!), brake light, notek, turret searchlight and 3 working machine guns. With lights your tank will look alive!