seeker333

It's not the type of tubing that makes a frameset more rigid, rather the tubing diameter (and to lesser extent tubing wall thickness) that increases rigidity. Diameter of downtube, top tube, stays, headtube and fork steerer tube all play a role in load-handling capability of a loaded-touring bike.

Your 180lb weight is not particularly high in these days of normalized obesity. The biggest issue in your bike's setup is the huge basket on top of the rear rack. This is the worst way to carry a load on rear to prevent shimmy (front loading has a lot less to do with shimmy than rear). For best results, use panniers which hang from the rear rack rail. This change alone should fix the shimmy problem. Leave your headset alone, it has no bearing (unintentional pun) on shimmy. Also, panniers on the front are easier to handle than loaded basket.