Thanks for your answers.
@monarda I sometimes measure the inside temperature of the bread. It is usually around 97˚C / 206.6˚F.
DH doesn't like the very crispy crust you get when you bake it at very high temperatures. Therefore I bake it at a much lower temp than the usual sourdough bakers do. That should work if it is baked longer.
@sparkytheop I use an electric oven with a fan.
The times that I mearured the inside temp and left it in the oven until 98˚C, the moist didn't disappear. But your 210˚F is 98,8˚C, so I should aim for 99˚C.
@ROF Expat It is not a no-knead dough. I knead very well, by using a strong hand mixer and mix for at least 10 minutes. I also shape the loaves afterwards by stretching and folding. Lately when trying to make improvements, I made bread with 100% wheat (still 50% wholegrain) and then I noticed the dough was getting much firmer during the kneading. Especially when I only mixed flour and water first and left it for an hour. Kneading in the salt and later the yeast after that hour had past. That was a method described in a sourdough book. So it is not (only) the lack of firmness. It happens both with firm and less firm breads.
My loaves are 500-550 gram flour. I use normal Norwegian baking flour which has a pretty high gluten percent. I could use typo 0 or 00 flour, but that is less healthy as those contain only 40% of the whole grain. The flour I use has 85% or so of the whole grain. I think there is no point in making the perfect bread using only white, unhealthy flour.
Today when baking, I put the bowl of water in the oven, but removed it after 20 minutes or so, slipping out the moist as well.
I don't use 60% rye or spelt, but I might have used up,to 30-40% together. Usually I only use 10-20% spelt or rye. But I know I get better results (bread with more air bubbles) by using wheat. The humidity problem is also there when I only use wheat.
I always do the second rise in the bread molds, but depending on how firm the dough is, it will cascade over the sides of the mold. So often the bread is put into the oven prematurely, before it goes out of the baking molds. But yesterday I baked a fruit bread that was puffed for a long time and became a very open bread. Still, it was very moist inside.
I do not use a stone or a steel, but I have metal bread molds that are not preheated. Maybe I should try making some not-sourdough bread without any molds. Just right on a metal baking tray, like I used to do in the past. Alternative is making sourdough with puffing baskets, but I find that more hassle. And they tend to collapse as well.
Next time, I will mix the dough much longer with lower hydration and not add more water. I will use only wheat flour and measure the inside temperature after baking until it is 99˚C. I will do the latter also with the bread that is currently in the oven.
Thanks for all your inputs. I sounds like the end temperature is too low, so I should bake it longer, and measure. And my dough has too high hydration.