I was mostly following a tutorial by a german guy of YouTube called "the bread code". I got some wheat flour from a local mill with 13% protein content.

For optimal hydration he had a smart trick where you make 5 sample doughs with 50g flour and then 60, 65, 70, 75 and 80% hydration. You just mix the flour and water and wait 15 minutes and then see which has the best window pane. Mine was at 75% but i went with 70 cause lower should be easier.

I think what my flaw was that i stopped adding dough strength during the last 4 hours of fermentation, so my first change will be continuing coil and folds for longer and see if that fixes my issue.