Perfect, after years of dealing with client chosen solutions for page builders in WP, this is the one I will be recommending they use in the future.
It does everything the WordPress way and uses the same structure as the customiser so you can see your edits live, a bonus is being able to use the same device type previews the customiser uses so you can seamlessly swap between mobile / tablet / desktop view while building your page.
Well done guys this is a triumph, look forward to getting my hands dirty and building a few custom extensions.
Final words: You will love it :-)