White sourdoughbread with a little spelt (hydration 71%)

500 grams of white levain 100% hydration
500 grams of organic roller mill bread flour 11% protein
150 grams of organic stone ground bread flour 10% protein
100 grams of organic stone ground whole grain spelt flour ( I like a bit of whole grain in my white bread)
465 grams of water
18 grams of organic sea salt

Mix everything but the salt by hand for a couple of minutes. Autolyse for one hour. Add salt and knead by hand for two to three minutes.
Put the dough in pastic box with a little oil. Stretch and fold and rest for 30 minutes, repeat and rest for 30 minutes,
repeat one last time and rest for one hour. Pre-shape two loaves and rest 10-15 minutes. Shape batards and place in benettons.
Rest overnight in refrigerator, about 11 hours. Take out the dough and let it rest in room temperature for about 45 minutes.
Then place in pre-heated oven with baking stone. Steam for the first 15 minutes.

Rather pleased with the crumb which came out soft and airy. Lots of holes, but not the really big ones.
I´m finding it hard to achive big holes in the crumb whith the flour I am using. Maybe the flour is too weak?
We don´t have bread flour with 13-14% protein content like you do in the USA. 11% protein is considered strong here in Sweden.

Didn´t get much oven spring with this bread. I´m gonna have to work on that. I find it harder with higher hydration dough.

The crust is rather thin, would like it a bit thicker. Wondering why I get those cracks on the outside of the crust?
I don´t normally get those with other recipes. Also getting some pockets of air between the crust and the crumb.

Any help and ideas would be much appreciated. I hope you can understand my "english".