As for the well respected builder(s) telling you to use more dirt and less rock… their intentions are right however their wording is wrong. What this specific builder might be referring to is the basic design principals for sustainable trails. By following them then you will be able to use more dirt and less rock. However when people are adopting trails on the shore, like you Ian. Then you don't have the real ability to cut a sustainable trail and are forced to do more trail hardening by using rock and wood.
Talking about creating sustainable trails on the north shore that follow basic design principals recommended by IMBA. Well in my 9 years of riding on the 3 local mountains probably only a small handful of trails, and I'm talking less than 10%, have the chance to be sustainable and long lasting without heavy maintenance. What do I mean? The most of the trails on the north shore (and I'm talking about original trail cut) are technically speaking, shit. Thank god for local builders who so lovingly put their time into these trails to maintain them.
That's one of my pet peeves, this whole sustainable trail mantra. All this talk about rebuilding the trail so it's sustainable. Yet I never hear the mention of what makes the trail truly sustainable.
What makes trails truly sustainable is continous trail work. This is fixing the problems like clearing windfall, fixing holes. and so on. Without doing that a trail no matter how it is built will degrade over time and become shite.