This looks great! So glad the output is MJML. I will have to see how I can embed it in the project I am working on as I have been wanting to add the feature for clients to build out their own emails or customize some of the standard templates. It will at least be a big help in my own admin UI to start.
Amazing to hear :)
If you ship your app with it, send a link — happy to feature it on the showcase page. Also, if you have existing HTML-based templates, try the @templatical/import-html package to convert them to Templatical JSON true — should get you a reasonably close starting point.