Thanks Johannes, for checking out the app :)
The "Design It" button is rendered when the page loading is finished, I will check if we can show a progress icon there until the button load is done.
In cart page, there is a link "View Custom Product". This link exists for all the customized items, if you click that link then you will see the image of your customized product.
Please let me know if you have any other queries / suggestions.