Concrete base problem in garage shop. Any construction folks?

You have to have some kind of binder if you want to get compaction. In ND a common fill material is what is called Class Five.

https://www.magnusonsod.com/product/class-5/
You note that this has both fines and clay in it. The suggestion of mixing in some sand is a good one but if you could find some clay, the resulting subbase will compact to something resembling concrete using a plate compactor in no more than 6" lifts.