I have to second the geotextile. Several years ago I put it down under part of a patio. Note I said part. I ran out and didn’t use it on another section. Other than the fabric the whole patio was done the same way.
Fast forward 15 years. There is some settling in the area without textile and the area with it is dead flat.
It’s not so bad that I’d take up the uneven area to fix it but I’ve use geotextile on every project since then. It’s not expensive.