Insulating a metal shop?

I recently got my 30x50 square tube metal shop put up and am currently going through sticker shock on insulation. Not only that but researching it is making my head spin. It seems like no matter what I read, everything is always wrong with every form of insulation. I'll admit I have zero experience with this topic.

I'm much more concerned with keeping the heat out during the summer than I am keeping the heat in during winter. We have hot summers and mild winters here. I'm looking at polyiso foam board at Home Depot. 1 1/2" thick at an R value of 5.78. Was thinking about applying a double bubble radiant barrier to it while keeping a 1/2" gap between the foil and the metal siding. Would probably cost around $3-3.5k. What scares me is potentially spending all that money, doing all that work and having it still be hot as hell during the summer.

https://www.homedepot.com/p/R-Tech-...-Rigid-Foam-Board-Insulation-320817/202532855
https://www.homedepot.com/pep/Everb...CL3TestB-20550232742--&gad_source=1&gclsrc=ds
Does anyone have any recommendations?