Hey joe_l,
thanks so much for taking your time to check on my topic. I guess I could try to decrease the width, but I also think that it's about the "nature" of the curve, so probably the two lines would just be closer together and still behave as an (smaller) area. A vector shouldn't have such borders as shown in the images I attached. So I guess something maybe my tool selection (pen tool) is wrong.
I'm new to print stuff, but I guess creating a cutting line for printing company is quite common, so I wonder why I have this issue at all. I hope to find a solution, as the deadline for the project is approaching 😑