Sourdough tang comes from two acids your starter's bacteria produce: lactic acid (mild, smooth, yogurt-like) and acetic acid (sharp, vinegary, assertive). Fermentation conditions shift the balance between the two.

To increase sourness:
Extend the cold fridge retard well past overnight -- 24-36+ hours.
Use your starter a little past peak (starting to sag, smells more pungent) instead of right when it domes.
Use less starter, 10-15% instead of 20% -- slower rise gives the bacteria more time to build acid.
Add 10-20% whole wheat or rye flour -- whole grains buffer the dough, letting more acid accumulate before the bacteria shut down.

To make it milder:
Use starter right at peak, keep the cold retard short (or skip it), use more starter for a faster rise, stick with white flour.

Change one variable at a time. Stack several changes at once and you won't know which one actually moved the flavor.

Making Sourdough Bread