For the softest texture, keep the dough slightly on the wetter side and avoid adding too much extra flour during kneading, and ferment in a warm spot so the sourdough can rise efficiently without becoming too tangy; if your kitchen is cool, extend bulk fermentation or proof the shaped loaf in a slightly warmed (but turned-off) oven. Enrichments like milk, butter, and a little sugar all help softness, but don’t overdo them or the dough may struggle to rise, and always make sure your starter is very active before you begin, as a sluggish starter leads to dense bread. Storing the cooled loaf in an airtight bag or container at room temperature will help maintain softness for several days, and you can refresh slices by briefly warming them in the toaster or oven. Once you like the basic recipe, experiment with adding a bit of tangzhong (a cooked flour paste), honey instead of sugar, or swapping a little of the bread flour for more whole grains while watching hydration and fermentation times.