looks like you know what you’re talking about. I have the same issue with the actual released sage (and many more → Sage 9 - yarn run build:production - No errors, but missing CSS / https://github.com/roots/sage/issues/2017#issuecomment-361054297)
Maybe you already have a solution for this?
Because the actual webpack.config.js changed a lot since you describe your solution.
Thx