I was hoping you would get an answer from someone out your way, but you haven't. Here's the best I can do.

Your plant list likes well-draining soils, but I wouldn't worry about the 'lean' too much. Some actually like and/or will do fine in a more standard soil, as long as it drains well. They don't like their feet wet. And I wouldn't worry too much about a specific 'soil formula', especially for so large a bed.

A two part answer.

First, what to fill the bed with isn't too difficult. Start with decent garden soil or purchased topsoil, and pea gravel or other coarse inorganic material (I would avoid using too much sand.) First pea gravel or whatever, then a layer of mixed gravel and topsoil, then plain topsoil. You don't want to create a distinct "boundary" between the layers. Just a rough gradation from bottom to top, if that makes sense.

Which brings us to the big potential problem... The existing boundary. You said that your soil surface is rock-hard. I don't know your soils where you are, but I suspect you have to find some way of breaking that surface up some, unless your raised beds are going to be two feet deep or more. (wince. too much work and expense.)

Depending on what makes your soil rock-hard, clay or other material, you may be able to rototill, or some other method might be easier. Check with your local county extension or state university agriculture/horticulture department, or even the local garden club. At least the first two may actually have websites that can help. If not, pick up the phone.

It may be made better just by putting down a layer of humus or compost first, and letting that do its work... Can't tell without knowing what makes the soils in your part of Idaho so hard.

Hope this helps at least some.

idahoan,
You can click on the top left cornor, to, "Other Forums", go to "Winter Sowing",lots of great people over there can help you out, they are a really nice group.
cAROL