Its a shame that numbers 6, 7, 8, 9 and 10 would have had little effect
Because it grows with speed it's definitely something that's rotating. The frequency will indicate if it is engine or gearbox related or driveshaft/unsprung related.
If it's high enough frequency to sound 'buzzy', then its tranny/engine. If the frequency changes with gears then its something pretransmission, so engine or torque converter etc. If its dependent on road speed it could be some other part of the transmission or driveshaft. I know this isn't your symptom, but I'm just putting that here to paint the whole picture.
If its a relatively low frequency, and its dependent on road speed, it will be diff/driveshaft/unsprung related, more likely unsprung. Sounds like this is what you're describing.
Because it reduces under load, I am thinking its unsprung. Basically at low drive load all the bushes are unloaded in the longitudinal direction so the vibration can build amplitude and everything can 'flop' around. When you're braking or accelerating everything is getting held to one side so its all firm. Bushes are designed to have this flex/movement.
Driveshafts and CVs are pretty much on the axis of rotation, they need to be pretty farked to cause noticeable vibration. If they were bad, you'd see noticeable movement when you spin the wheel around, or be able to make heaps of movement when you pull on them hard.
Seeing you've changed shafts and diff, and tyres, and bearings, then i would check for a bent or otherwise misaligned hub or dirt etc stuck inside one of the ventilated discs. There has to be some unbalance somewhere, and its up to you to find it. Spin each wheel and make sure they are running true. Check all nooks and crannies for dirt. Seeing as the bearings where done prior to the vibration starting, its possible they were not pressed in fully (not sure if those are pressed ones) or were manufactured badly. This will show up as wheel misalignment when spun. Or play. Same for foreign material between some of the bearing/hub surfaces.
I had a set of tyres once that for some reason wouldn't balance. Like they ballooned out when spinning or something. Could try someone else's wheels to totally eliminate them as a cause, but you should have noticed a difference when you swapped them all around.
Good luck, vibrations suck