How to achieve R-21 insulation in 3.5 inch exterior wall
I had a similar wall that I insulated exactly (well, almost) the way you're describing.  Here's a cross section of the wall.  The left side is towards an unheated workshop.

I don't think I used 1.5 inch XPS because it would have caused the sheet rock to bulge slightly.
Another option is what they call an R13+5 construction.  With this technique, which can be used for new construction or when you're tearing down to the studs, fills the 3-1/2&quot; wall cavity with something like Rockwool (R13), then fasten foam board insulation with an R-value of 5 to the outside of the wall, before your house wrap and siding.  The chart excerpt below is from a local building code information brochure.