Need help with a switching layout!

Good switching plan. I've done similar ones for a spare bedroom.

I took your sketch and set up an RR-Track 5 file using the generic 2-rail library. The curve radius is 36" (O-72 in 3-rail speak.) The turnouts are all #4's. Track spacing is 4". Benchwork depth is 32". That should leave you enough space to set your industries, while allowing you to reach everything.

My first HO layout as a teenager was an L-shaped switching layout, because that was all I had room for. One thing I did that worked pretty well was to include a switchback down to a hidden lower level where I could "stage" cuts of cars. I "split" the grade to keep it manageable.

Another possibility to consider is a separate detached second level, maybe with just a couple long parallel tracks well above the main layout, to use as easily accessed storage for additional cars that can be rotated onto the main layout for variety.

Wow! Thank you so much Matt for doing that rendering for me!

Matt, is there enough room for a runaround in the bottom left corner?

Unfortunately, not in a 12x8. I did a 12x12 experiment which had a run-around and could support additional spur space.

Here is the room I am working with, 12'x8'

What if I had an around the room layout, what might that look like?

I'm open to just about anything, as long as it's scale.

Looking at your room orientation, here's a variant that gives a little more "main line" with more complexity (which is a good thing for a switch deck.)