Thick vs thin decoction are about enzymes - they stay behind in the liquid. Pull the liquid and you denature the enzymes.
Decoctions don't extract tannins b/c the pH is correct. The pH danger w/sparging (more fly sparging) is your buffering capacity gets washed out, so the temp matters far more to change the pH. With batch sparging, it's less of an issue, especially if you're treating your sparge water w/salts. So use hot, but not boiling water, and you'll probably equalize ~160ish, no problem.