Yes, I quite understood the stone was always parallel to the slope, but it’s movement can still be seen as a combination of a vertical distance and a horizontal one. Like going up stairs, for instance. The angle of elevation of the staircase is dictated by the ratio between the risers and treaders. The steeper the stairs the longer the risers, relative to the treaders.
So, if you progressed up your slope at the same rate as you did on the level you must have been increasing the “riser” element of your effort, presumably sub-consciously, by pressing your levers down further. It would be only 12.5% more, so you might not notice. The fact that you didn’t notice absolutely proves my point that your method is the only one that can allow a situation where the ancients could employ a team of the same number of people to shift a stone for the whole journey, and wouldn’t have to bring in reinforcements for the steep bits. Julian Richards postulated to the Royal Academy that the Stonehenge builders might have used a workforce that was variable in numbers. So far as the transporting of the stones is concerned I think his suggestion can be dismissed. You’ve already managed to put the greased trackway theory onto a slippery slope…

About the brakes: I was wondering, if you had an arrangement whereby you lashed some brake levers to the back of the stone and dragged them behind, with one end scraping along the ground, then when you stopped, and the stone wanted to slide back, the levers would dig into the ground and stop it. The brakeman’s job seems a bit hazardous, unless he can do it from the side. Even if you still need to use a brakeman, this arrangement would at least give an extra layer of safety for him.