After hundreds of thousands of miles on this platform, in a couple different vehicles, and having fought balance issues constantly, here's what I can tell you:
The steering system is very sensitive to imbalance, the slightest amount you will feel in the steering wheel.
Tires flat spot - you will always have some amount of vibration until you have 10+ miles at interstate speeds, even on perfectly balanced tires.
Most tire shops don't have cones to properly fit the Toyota 106.1mm wheel bore and do a poor job of balancing.
Aftermarket wheels without a 106.1 center bore need centering rings to get consistent centering on the hub.
Many balancers are out of spec
Most techs doing balances don't do a good job - I have found this to be the most important factor. I have a tech at my local discount tire that is the only one I let balance my tires. He does it perfect every time, even on big, heavy, torn up 305/70/17 MTRs.
Other than getting a very good balance, the only thing that has any hope of improvement is to replace the preload springs in the steering rack, outlined in a Lexus GX460 TSB, which requires removal of the steering rack.