From personal experience, I don't think adding concrete will gain anything. Seeds will continue to collect in the stone, they'll sprout and you're back to square one.

I think spraying the weeds as they sprout, or applying a pre-emergence, is the only thing that will really work.

Use a propane torch to burn off the weeds every week or two.

Everyone assumes weeds root from beneath, thus the felt etc. They will grow on the top as well. I might not go around with a torch but if your area hasn't gone chemical free like ours a chemical spray like Round Up will do the trick.

Remove the pebbles if possible. Lay down a sheet of landscaping fabric and then place the pebbles on top of it. The landscaping fabric will allow water to penetrate the ground, but it will not let weeds grow.

The landscaping fabric will allow water to penetrate the ground, but it will not let weeds grow.

I disagree,

Weeds will grow on the soil that settles above the cloth

The quikrete = a disaster in the making. Unless you love the look of crumbly concrete everywhere with weeds growing through it.

Landscape fabric + anything but especially stones = a disaster in the making. I don't care what you do, you're getting weeds in the stone, as well as dirt and debris and funk.

Unless there's a compelling design reason or the client can't be dissuaded, I never ever use stone as a "mulch". I was in maintenance before I was in design and you always be fighting nature. And losing.

Honestly, the torch is the best idea. You're not looking to create blackened scorch marks everywhere, you're just heating the plant enough that water in the cells boils and bursts the cell walls so the plant dies. They sell one that I think is called "the Weed Dragon" or something. Not a bad way to go.