In the half-black-half-white circle icon menu in the Layers panel, it's called "Solid Color". Once you add it, it is called "Color Fill". I've never understood it, I wish Adobe would fix it.
I love what you've done. Where are you seeing banding?
Download the PSD
I tried a bunch of different artistic filters, and found that Paint Daubs sort of kept some shape, but got rid of the texture sufficiently. However, try them for yourself, you might find another one you like better.
Ok, so if you turn all those layers off, then turn them one again one at a time from the bottom up, is there one (or more than one) which is an obvious culprit for the banding?
This is an odd one, for sure. As Sam said, everything in your specs suggests it should run fine. Out of interest, is it the same if you're editing an image which is on the SSD?
Is it the same if you push the Whites slider all the way up?