"I added two proportioning valves for the front brakes, one for front-left, one for front-right. When I adjust them, I just ensure I turn them the same amount. "

When adjusting them you might turn the ABS off first so that it is not compensating and you can better detect when the wheels are locking up.


"My only brake issue, which is a major one, is that the pedal effort is WAY too high."

Have you tried changing the brake pads to a more aggressive material?