All the methods your friend told you are reasonable. If you use any one alone, it will have certain effect, but it will not satisfy you, because it will not make you feel that summer is not too hot.
There are three basic ways of heat transfer, namely heat conduction, convection and radiation. The sun shines directly on your color steel composite wall, and the temperature can reach more than 80 degrees, so the inner wall will be very hot through heat conduction. This kind of wall should have good thermal insulation performance, but it is too thin. Thickening is possible, but it is not easy. You can add it outside and inside. This method is the simplest.
Convection and radiation exist in heat conduction. To solve this problem, it is necessary to build a simple house, preferably sealed, set up a shed or plant flowers.
In short, a multi-pronged approach is needed, and a simple house is not easy.