FYI, I just shared your project on Product Hunt: https://www.producthunt.com/posts/printable-mockups
I think the design community there will find it really useful!
One suggestion: maybe add an empty block for a placeholder to make space for notes.
On GitHub found only few Inkscape add-ons for creating interactive mockups.[1,2,3]
[1] https://github.com/jonathanmann/inkscape_app_mockup
Also, might be due to inexperience in the field but why import/export in JSON ?
They could use a different file extension and even gzip the file as well. I often use line separated JSON+GZ as an interchange format for records.
For all of the electronic communication in my company, once things get to a certain level of management, they want to see stuff on paper.
You could probably turn this into a product if you were to print out notebooks where pages were the templates.
If it’s a startup project just use whatever your most familiar with and change it when/ if it becomes a problem.