Hey, you could try a tiny splash of tannic acid—like the stuff in oak barrels—mixed into a water‑based stain. It reacts with the lignin in the wood to deepen the hue, but it’s pretty safe and won’t leach out. Just stir a teaspoon of the acid in a cup of warm water, let it cool, then dilute it with a bit of your stain and apply. The reaction is subtle, so the color deepens gradually, and it stays stable once it dries. Just keep a small test strip on a spare board to make sure you like the tone before you go full scale!