I see where you mentioned the bent rims in the original post so I did miss it. IKO look no further than the wheels. Human eye cannot see the deviation.

New wheels typically need 1/4oz weight to balance them, I would guess despite Wheel Specialists best efforts your rear wheels require a full 1oz just for the bare wheel.
Before debuting a unknown wheel on your car, always have it spun on a balancer & observe the up & down "hop", as well as the side to side movement to see what you are dealing with. Especially with new wheels since often they are non-returnable if a tire has been mounted.

Well today I went back to the tire and wheel shop to have the wheels checked again for out of balance. I've been battling the same vibration issue at 70 mph and higher.

The shop says all 4 wheels roadforced balanced less than 10 lbs and all is good as far as the wheels are concerned. So that leads me to believe that the suspension or steering is the culprit. Since I've observed no slop in the steering at all, and it is very crisp, I believe the LCA bushings or balljoints must be worn.

I have not replaced any suspension components other than the rear shocks after I bought the car last year. Front shocks rebound fine, no leaks. LCAs look old. Any way to tell for sure? If I do LCAs, should I also do the track arms?

You'll probably find that the tuning fork bushes are worn and tbh there's no real way to just visually check them. When i replaced mine the old ones "looked fine" but you could virtually bend them in half with very little effort at all.