Given the construction method of the wall and that you like the idea of foam and it is DIY so the time spent on the project isn’t a factor. I would do the whole job with 3” thick foam. I got a load of free used foam when I did my garage workshop and it was old DIY construction with stud bays of all different widths. I cut and fit the foam for each opening sometimes piecing it in. I got it fairly tight and when done I used great stuff spray foam on any cracks and joints. I cut mine with a sawzall. But any saw with a long blade will cut it like butter. There is no waste as you can piece it in to use it all up. Then inside drywall or peg board or even plywood and paint.