"only a certain steepness of angle can be achieved before the sides slump and collapse. therefore by trial and error most cultures found the maximum achievable angle(for the given building material) was....and most would have opted for something approaching the maximum because this would reduce base area and thus amount of material required."

I think that's right. EH's consultant engineer said Silbury's slope was at, not beyond, the optimum before soil-slip would happen. Silbaby has an identical slope.

It's reasonable to suspect that's the slope they would have aimed at since it's the most efficient in terms of amount of material versus height, as you say.
OR that in some cases they overdid it, in which case erosion would happen and the slope would reduce to the optimum one over time.

Soil slip is only a factor with soil and loose material of course. Is this a reason to believe that covering it with soil or loose chalk was in the job specs for Silbury from the start? Probably - and the turf which then developed is arguably the one feature that has ensured the main structure and profile of Silbury has remained unaffected and unaltered for millenia, IMO. Did they know this as well? I reckon so and that they were a bit damn clever.