I'm pretty new to sourdough. I've been experimenting with different starters and have made some nice tasting bread with the familar sourdough chewiness - but I just haven't been able to get them to taste very sour. Until today. I made some dough and left it in the fridge to ferment for a few days - I've lost track of time with the holiday but I think this dough has been in the fridge for 3 days. I shaped it cold and then let it warm up while I got my oven preheated up to 700F (I have to trick my thermostat - otherwise 550F is as high as it will go) and I have a 1/2" steel in the oven which really makes a pizza bake quickly at that temp. I know that with pizza, less is more - but I can't help myself and I put a ton of toppings on. Pepperoni, onion, red peppers, olives, fresh basil, pecorino, mozzarella. This didn't puff up as high as I get when I use commercial yeast instead of or added to sourdough - but it really has a lovely crunch and tons of sourdough tang to it.
and yours looks grand. What makes sour is both warmth and cold (92 F for a short time and 36 f for a very ling time, high hydration when warm and low hydration when cold, whole grains but really bran that acts as a buffer allowing the LAB to continue to make acid at pH's much lower than normal and some lab make more acids than others. Having a starter that is cultured to have more LAB and less yeast in it than normal and in the levain as well,,will also produce a bread with more sour since LAB make acid and with fewer yeast in it the dough takes longer too proof so the LAB can make more acid but if the pH gets too kow the acid production will stop and that is where the bran come into play.
Goof luck with your sour quest.