Bread measures 95°C in centre but seems underbaked?

You really should wait till the loaf is cool to the touch. I wouldn't slice until at least 3 hours after it came out.

Apols if I'm wrong, but the load looks a little dense, and that dark patch in the middle at the base of the loaf looks a bit solid... perhaps it's worth looking at the proving time/temp or the amount and activity of the starter?

My loaves get stretch/folded for about 4 hrs too; but they then get 4/5hrs more in total at room temperature before they are fully proved and ready to go into the fridge for an hour before baking - so quite a bit more than your 3hrs from chilled...

At the end of the day, any sourdough bread is GOOD bread, and yours looks lovely :)

 I love the colour of that crust :)

I feel your pain! This reminds me of trouble I was having when I first started with SD - my crumb seemed heavy, and the loaves would spread relentlessly after I turned them out - to the extent that my bread was like a rather thick pizza! I’m not saying that you have that same problem at all; just that I know it can be a bit frustrating when you’re putting the effort in and not getting the results you want.

I think that was when I started refrigerating the fully proved loaf (yes, still in the banneton) before turning it out to bake - basically just to stiffen it up and stop it spreading so much... but IIRC I also made some other changes, not least changing the brand of flour that I was using: paradoxically, to a less ‘strong’ one (lower protein level) but a higher mineral content.

i have to be honest, I never had much luck with overnight refrigeration of the dough during proving. I know it produces some great results for others, but for me the yeast seemed to get ‘grumpy’ and the bread took *for ever* to rise after I took it back out - and then (I think because the whole process had taken so long) the gluten seemed to be really weak and the rise was very unsatisfactory (and also had really big holes like yours). Maybe it’s because my fridge is a little too cold? Anyhow, after a couple of attempts I never really persevered with it :)

If it possibly works with your schedule, I’d encourage you to try a loaf straight through without the chilled proof - just to discount this from the equation. Personally what works for me is to mix the levain up just before I go to bed at about 10:30pm; then start the autolyse (using hot water) when I come down to make the coffee at about 6:45am. I also give the levain a bit of a stir at that point which perks it up a little. After about 15mins I give the autolyse a bit of a knead to encourage some cross-linking of the gluten. Then after another 15-20 mins I add the salt, give it a bit more of a knead, then leave it to relax for 10mins before mixing in the levain (these days I’m normally lazy and use the KitchenAid to do this, to save the mess). Then I do 5 or 6 rounds of vigorous S&F 20-30 mins apart, really stretching it as far as it will go. That takes me up to about lunchtime. Then I leave it alone to prove for a couple of hours before I carefully shape it (building up as much tension as I dare) and put it in the banneton. I know when it’s fully proved when it’s completely filled the banneton - usually about 4pm to 5pm - at which point I just put the whole shebang uncovered in the fridge for 45mins to an hour before turning out, slashing and baking :)

I’m not saying that will work for you, but what I mean is sometimes it’s good to try a different approach to see whether it gives better or worse results than what you’re already doing :) it certainly was a bit of an eye-opener to me to switch down to a ‘weaker’ flour and get stronger gluten.