I like the simplicity of the UX, I would find it much more usable open source so I can adjust the template easily.
I wrote a similar thing for my board game site [0], using yml files as data sources with jekyll includes to generate the html and a html2canvas conversion to make printable images of individual cards that I have to manually put on a print-sheet -- not quite as streamlined as this, and I would love to learn from the rest of this person's flow architecture on how to generate a pdf, make it downloadable, expose the json editor. I know I could learn each of these on my own, but this is so close to my usecase it would be nice.