HosLaw,
I'm same age, roughly same height, and hover around 200 lbs. I cam back to riding bikes after a 10 yr break and the last bikes I had ridden were an XR100 and RM80. The two bikes that come to mind that are (a) close to bullet proof, (b) "not overly aggressive, but also not a wallflower" and (c) around $2k used but in good shape would be KDX220 and the XR400. The only thing that makes me unsure is "CA high & low deserts mostly"...I've never ridden in a desert, high or low, and I know that is where the big bore 2strokes and 4strokes shine.
Personally, I am riding a KDX220 and love it. I'd describe my riding style about the same way you did, but I am riding tight trails in the Appalacian Mtns.