Yes Skeletonization should propably not be used here. I was too focused on getting “better” results with this. I thought if i see no edge gaps here i can start to fill it (before Skeletonization). But i think the whole “fill” approach is propably weird as i should propably just use the picture before Sobel as you have said.
I will now try to get rid of the noise again by using “Opening” on the picture before Sobel. Afterwards i can hopefully turn it to binary and Skeletonize.
Just to make sure that i understand Skeletonization (and your critcism) correctly.
Would Skeletonization give me approximately this result? (edit: i forgot 2 lines at the top left)
Edit: Thanks for your patience btw. 