Yep I think the relationship between renderers and mustache is the key hurdle to understanding Boost theming. Previously you could do a lot more with layout files but these seem to be a lot more restrictive within Boost development. They now seem like just a place to call your template and a few other functions.
I've gotten over a couple of hurdles recently and now have an understanding of how these fit together but the Moodle docs seem very sparse on explaining things.
It's be really useful to have a doc that had richer examples ie. we're going to change the layout of the cog icon menu and move it to the top of the screen. Here's how we'll go about that.
This would give a clear understanding of the major elements of a Boost theme and how they interact. I might write one myself to solidify my knowledge as I work through developing our theme.
There certainly seems like there's a gap in the market for a Boost development training course (could even be delivered at distance).