Hi there, Masaru! You're welcome to try extending the cold fermentation time a tad if you'd like but you also want to be mindful that the dough doesn't become over-proofed by doing so. For a more sour flavor, you might consider increasing the amount of whole wheat flour you're using or maybe even incorporating some rye flour. Whole grains are a  great way to add flavor. It also sounds like the dough is getting a bit over-proofed, which is indicated by the spreading and difficulties with scoring. We'd recommend keeping a close eye on your dough while it's resting at room temperature and maybe shortening up those rise times. We hope this helps for future baking adventures! 

• Reply