All I can say is if you need airbags then you're overloading the Hilux! Load-leveling bars should even out the load between the trailer and the Hilux. If they don't, then something is overloaded. I tow 2.5 tonne behind my Hilux and the load-leveling bars are sufficient to equalise the load between the Hilux and the caravan even with 230kg tow ball weight. Airbags have a known history of cracking chassis and I expect it's because they encourage overloading and the vehicle is not designed to have them.
The brakes issue is also part of the whole scheme of things... You must have electric brakes adjusted to equalise the amount of braking between the vehicle and the trailer. One way to check this is to put your Hilux and trailer, fully loaded onto a weighbridge and you might be surprised how much your combination is overloaded. This can also demonstrate the weight distribution between the vehicles. If you don't do this, somewhere down the road the transport authority will do it for you and you won't enjoy the outcome if you're overloaded.
As to brake proportioning valve I understand all vehicles have them but as you suggest, airbags spoil the whole issue. If your vehicle/trailer combination is correctly loaded, the proportioning valve will be set at the appropriate level.
Hope this helps
My 2013 auto D4D DC SR5, prefilter, 3" exhaust and Chip with egt probe, stock SR5 rims with Toyo Open Country AT2 265/65R/17's