The grass roots aren't the issue, Without seeing a 360° view, we don't know how close the nearest tree is. Some tree roots grow quite a ways to the sides, and can be hiding just under the soil.

For my last house, I dug about 6 inches down, and put a layer of gravel to fill up to the ground level. Pea gravel on bottom 3 inches and larger inch or so gravel on top. Then spaced the bottom ring so there was about 1.5-2in of space for air to flow in.

I also tried covering the inside of the stones I used with leftover high temp cement from making a blacksmithing forge, but it didn't adhere well to the stone and flaked off after a couple uses. it would probably do better on concrete retaining wall slabs.

I built it to last, for sure. But I wouldn't say I over-built it.

This kind of ring in OPs pic is probably fine for their purposes, but it's always good to be aware of things like root fires.