Using a 1 1/8” down tube with 8/5/8 wall thickness for a touring frame will not work well - at all. Before OS tubing became available because of the popularity of mountain bikes, Columbus SP tubing was a popular choice for a touring frame. It has 1/7/1 wall thickness for the main tubes and the blades and stays are decently heavy too. Another option is OS 9/6/9 tubing.
What happens with lighter tubing is that the frame will sway around when pedaling under load making things seem unstable. In the 70’s I built a frame with Reynolds 531 standard tubing that had essentially a 1/7/1 (actually 19/22 wire gauge thickness) with a 9/6/9 top tube and I was not pleased with how flexible the frame was when loaded. And like Andy I’m not a very big or heavy guy. I then made another touring frame out of Columbus SP and was entirely happy with the results when it was loaded down with panniers and other gear.