Your plan is do-able, with the key considerations being train length and turnout type (scratch-built or store-bought).
Here's a rendering of your idea with many scratch-built curved turnouts:
Design uses 6-inch squares.
Dark Grey is C55 ME concrete, red is C55 Atlas wooden.
Mainline doubletrack is on 1.25" centers with crossovers of #10 turnouts.
Yard & branch have min. #7 straight and many #8 36"/24" curved turnouts.
Visible mainline has 20" and 18.75" curves, while hidden dials down to between 12"-15" (which you can expand if you want benchwork to come out more into the room--I just wanted to show "minimals").
I basically just rendered what you sketched out, where you sketched it out, so if you want to expand the yard you could start the yard lead and the yard ladder about 18" earlier.
You've already thought about helices, so I thought I'd show what a "no-lix"ish design would look like:
By having the mains descend on long ramps of 2% grade, you could wrap back around and have a yard about 9" railhead-to-railhead (minus ply/foam would leave about 7" of space, plus "access holes" just in case you need to reach ramps.
(Like bdennis said, you need to take into consideration switch machines and anything else that protrudes down).
If you do go with helices, then I recommend designing so the yard tracks & turnouts are as close to you as possible.
I also included a programming track and two different locations for stub-end storage / staging for your mine loads.
Here's a list of all the turnouts I used in the design (last number being the quantity):
R10R40-30, N Fast Tracks R10R40-30 Right curved turnout 5.71º 4
R8L24-18, N Fast Tracks R8L24-18 Left curved turnout 7.13º 1
R8L30-20, N Fast Tracks R8L30-20 Left curved turnout 7.13º 2
R8L36-24, N Fast Tracks R8L36-24 Left curved turnout 7.13º 3
R8R30-20, N Fast Tracks R8R30-20 Right curved turnout 7.13º 2
R8R36-24, N Fast Tracks R8R36-24 Right curved turnout 7.13º 6
T10L, N Fast Tracks T10L Left turnout 6 13/16". 5
T10R, N Fast Tracks T10R Right turnout 6 13/16". 3
T7L, N Fast Tracks T7L Left turnout 5 7/16". 4
T7R, N Fast Tracks T7R Right turnout 5 7/16". 12
T8L, N Fast Tracks T8L Left turnout 5 15/16". 1
T8R, N Fast Tracks T8R Right turnout 5 15/16". 2
Y8, N Fast Tracks Y8 Wye turnout 5 15/16". 5
That said, I did this rather quickly as an approximation.
If I were to build this layout, I'd lay out the trackage for the mains, yards, etc., and then use tracing paper to make turnout templates (or use Fast Tracks templates that are "close enough") and build multiple-turnout fixtures that ensure smooth flowing curves.
Especially as you like big modern diesels
Narrowest point is still 30" so you need to be wary of layout height: both reach to back of branch line on top as well as to any trackage underneath.
The plan supports trains 5'-7' long (though through trains can be long as you'd like!).
Anyway, hope this helps give you some ideas for your space.
Cheers!
