SST, forget about trying to drybrush Tamiya acrylics; they just won't go there! You're better off to try just about any other paint.
Have you tried regular craft store acrylics? They come in a bewildering variety of colors, and with a little bit of practice, you can mix up just about any shade. You don't need to mix enamels into this...you'll regret it if and when you want to apply washes.
Your figure looks on-target for the base coat. I wouldnot however, go solely with the "wash" method. Don't get me wrong, its a valid technique, but the amount of folds in this figure's clothing begs for individual shading with a darkened shade of the field gray. Just add a touch of black--or you can even try panzer gray--to a small dollop of field gray, and with a tiny brush underline all the folds and anywhere where a shadow would fall. You can THEN try a very thin wash of raw umber, and follow that with DB'ing.
Or, for a different look--more dusty and faded--try a somewhat thicker wash of raw umber+white+a pinprick of yellow oils (yelllow oil is VERY dense!) to get a nice dusty tan shade. Wash that over the figure and then take a paper towel--NOT TISSUE!-LEAVES "HAIRS"!!!--and squeeze it around the figure to get most of it off the high parts, leaving it in the folds. Dont worry about it "destroying" the shading you'd already done; it will provide depth to these.
Good luck! EXPERIMENT!!!![Propeller [8-]](/emoticons/icon_smile_propeller.gif)