Concrete base problem in garage shop. Any construction folks?

I have used what's called "bank run" which is various sized gravel/sand specifically meant to pack. We have also used a morter mixed limestone, which is mainly used for filling pot holes in gravel driveways, and turns to a hard base with moisture. If you still need 7 or 8 inches of fill, I would look at topping off what you have with one of these.

Assuming your not pouring your own slab, consult your concrete guy. Some care more than others. I poured in high school and college, and usually uneven or non compacted gravel didn't hurt a pour. Would just take more concrete to fill the voids in say 57 limestone, vs bank run.

If your base under that has been there for 25 years, it's pretty solid.